enable prefer const (#5113)

2017-01-14 09:48:52 -05:00
parent 982850731e
commit 672adba9a1
177 changed files with 1862 additions and 1863 deletions
--- a/packages/babel-core/src/api/browser.js
+++ b/packages/babel-core/src/api/browser.js
@@ -29,16 +29,16 @@ export function run(code: string, opts: Object = {}): any {
 export function load(url: string, callback: Function, opts: Object = {}, hold?: boolean) {
  opts.filename = opts.filename || url;

-  let xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
+  const xhr = global.ActiveXObject ? new global.ActiveXObject("Microsoft.XMLHTTP") : new global.XMLHttpRequest();
  xhr.open("GET", url, true);
  if ("overrideMimeType" in xhr) xhr.overrideMimeType("text/plain");

  xhr.onreadystatechange = function () {
    if (xhr.readyState !== 4) return;

-    let status = xhr.status;
+    const status = xhr.status;
    if (status === 0 || status === 200) {
-      let param = [xhr.responseText, opts];
+      const param = [xhr.responseText, opts];
      if (!hold) run(param);
      if (callback) callback(param);
    } else {
@@ -50,8 +50,8 @@ export function load(url: string, callback: Function, opts: Object = {}, hold?:
 }

 function runScripts() {
-  let scripts: Array<Array<any> | Object> = [];
-  let types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
+  const scripts: Array<Array<any> | Object> = [];
+  const types   = ["text/ecmascript-6", "text/6to5", "text/babel", "module"];
  let index   = 0;

  /**
@@ -59,7 +59,7 @@ function runScripts() {
   */

  function exec() {
-    let param = scripts[index];
+    const param = scripts[index];
    if (param instanceof Array) {
      run(param, index);
      index++;
@@ -72,7 +72,7 @@ function runScripts() {
   */

  function run(script: Object, i: number) {
-    let opts = {};
+    const opts = {};

    if (script.src) {
      load(script.src, function (param) {
@@ -87,10 +87,10 @@ function runScripts() {

  // Collect scripts with Babel `types`.

-  let _scripts = global.document.getElementsByTagName("script");
+  const _scripts = global.document.getElementsByTagName("script");

  for (let i = 0; i < _scripts.length; ++i) {
-    let _script = _scripts[i];
+    const _script = _scripts[i];
    if (types.indexOf(_script.type) >= 0) scripts.push(_script);
  }

--- a/packages/babel-core/src/api/node.js
+++ b/packages/babel-core/src/api/node.js
@@ -31,10 +31,10 @@ export function Plugin(alias) {
 import Pipeline from "../transformation/pipeline";
 export { Pipeline };

-let pipeline = new Pipeline;
-export let analyse = pipeline.analyse.bind(pipeline);
-export let transform = pipeline.transform.bind(pipeline);
-export let transformFromAst = pipeline.transformFromAst.bind(pipeline);
+const pipeline = new Pipeline;
+export const analyse = pipeline.analyse.bind(pipeline);
+export const transform = pipeline.transform.bind(pipeline);
+export const transformFromAst = pipeline.transformFromAst.bind(pipeline);

 export function transformFile(filename: string, opts?: Object, callback: Function) {
  if (isFunction(opts)) {
--- a/packages/babel-core/src/helpers/get-possible-preset-names.js
+++ b/packages/babel-core/src/helpers/get-possible-preset-names.js
@@ -1,11 +1,11 @@
 export default function getPossiblePresetNames(presetName: string): Array<string> {
-  let possibleNames = [`babel-preset-${presetName}`, presetName];
+  const possibleNames = [`babel-preset-${presetName}`, presetName];

  // trying to resolve @organization shortcat
  // @foo/es2015 -> @foo/babel-preset-es2015
-  let matches = presetName.match(/^(@[^/]+)\/(.+)$/);
+  const matches = presetName.match(/^(@[^/]+)\/(.+)$/);
  if (matches) {
-    let [, orgName, presetPath] = matches;
+    const [, orgName, presetPath] = matches;
    possibleNames.push(`${orgName}/babel-preset-${presetPath}`);
  }

--- a/packages/babel-core/src/helpers/merge.js
+++ b/packages/babel-core/src/helpers/merge.js
@@ -5,9 +5,9 @@ export default function (dest?: Object, src?: Object): ?Object {

  return mergeWith(dest, src, function (a, b) {
    if (b && Array.isArray(a)) {
-      let newArray = b.slice(0);
+      const newArray = b.slice(0);

-      for (let item of a) {
+      for (const item of a) {
        if (newArray.indexOf(item) < 0) {
          newArray.push(item);
        }
--- a/packages/babel-core/src/helpers/resolve.js
+++ b/packages/babel-core/src/helpers/resolve.js
@@ -1,7 +1,7 @@
 import Module from "module";
 import path from "path";

-let relativeModules = {};
+const relativeModules = {};

 export default function (loc: string, relative: string = process.cwd()): ?string {
  // we're in the browser, probably
@@ -18,7 +18,7 @@ export default function (loc: string, relative: string = process.cwd()): ?string
    // Node presumes "." is process.cwd(), not our relative path.
    // Since this fake module is never "loaded", we don't have to worry about mutating
    // any global Node module cache state here.
-    let filename = path.join(relative, ".babelrc");
+    const filename = path.join(relative, ".babelrc");
    relativeMod.id = filename;
    relativeMod.filename = filename;

--- a/packages/babel-core/src/store.js
+++ b/packages/babel-core/src/store.js
@@ -15,7 +15,7 @@ export default class Store extends Map {
      return super.get(key);
    } else {
      if (Object.prototype.hasOwnProperty.call(this.dynamicData, key)) {
-        let val =  this.dynamicData[key]();
+        const val =  this.dynamicData[key]();
        this.set(key, val);
        return val;
      }
--- a/packages/babel-core/src/tools/build-external-helpers.js
+++ b/packages/babel-core/src/tools/build-external-helpers.js
@@ -7,7 +7,7 @@ import template from "babel-template";
 import each from "lodash/each";
 import * as t from "babel-types";

-let buildUmdWrapper = template(`
+const buildUmdWrapper = template(`
  (function (root, factory) {
    if (typeof define === "function" && define.amd) {
      define(AMD_ARGUMENTS, factory);
@@ -22,9 +22,9 @@ let buildUmdWrapper = template(`
 `);

 function buildGlobal(namespace, builder) {
-  let body      = [];
-  let container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
-  let tree      = t.program([t.expressionStatement(t.callExpression(container, [helpers.get("selfGlobal")]))]);
+  const body      = [];
+  const container = t.functionExpression(null, [t.identifier("global")], t.blockStatement(body));
+  const tree      = t.program([t.expressionStatement(t.callExpression(container, [helpers.get("selfGlobal")]))]);

  body.push(t.variableDeclaration("var", [
    t.variableDeclarator(
@@ -39,7 +39,7 @@ function buildGlobal(namespace, builder) {
 }

 function buildUmd(namespace, builder) {
-  let body = [];
+  const body = [];
  body.push(t.variableDeclaration("var", [
    t.variableDeclarator(namespace, t.identifier("global"))
  ]));
@@ -63,7 +63,7 @@ function buildUmd(namespace, builder) {
 }

 function buildVar(namespace, builder) {
-  let body = [];
+  const body = [];
  body.push(t.variableDeclaration("var", [
    t.variableDeclarator(namespace, t.objectExpression([]))
  ]));
@@ -76,7 +76,7 @@ function buildHelpers(body, namespace, whitelist) {
  each(helpers.list, function (name) {
    if (whitelist && whitelist.indexOf(name) < 0) return;

-    let key = t.identifier(name);
+    const key = t.identifier(name);
    body.push(t.expressionStatement(
      t.assignmentExpression("=", t.memberExpression(namespace, key), helpers.get(name))
    ));
@@ -86,15 +86,15 @@ export default function (
  whitelist?: Array<string>,
  outputType: "global" | "umd" | "var" = "global",
 ) {
-  let namespace = t.identifier("babelHelpers");
+  const namespace = t.identifier("babelHelpers");

-  let builder = function (body) {
+  const builder = function (body) {
    return buildHelpers(body, namespace, whitelist);
  };

  let tree;

-  let build = {
+  const build = {
    global: buildGlobal,
    umd:    buildUmd,
    var:    buildVar,
--- a/packages/babel-core/src/transformation/file/index.js
+++ b/packages/babel-core/src/transformation/file/index.js
@@ -32,9 +32,9 @@ const INTERNAL_PLUGINS = [
  [shadowFunctionsPlugin]
 ];

-let errorVisitor = {
+const errorVisitor = {
  enter(path, state) {
-    let loc = path.node.loc;
+    const loc = path.node.loc;
    if (loc) {
      state.loc = loc;
      path.stop();
@@ -69,7 +69,7 @@ export default class File extends Store {
      // All the "per preset" options are inherited from the main options.
      this.perPresetOpts = [];
      this.opts.presets.forEach((presetOpts) => {
-        let perPresetOpts = Object.assign(Object.create(this.opts), presetOpts);
+        const perPresetOpts = Object.assign(Object.create(this.opts), presetOpts);
        this.perPresetOpts.push(perPresetOpts);
        this.buildPluginsForOptions(perPresetOpts);
      });
@@ -125,7 +125,7 @@ export default class File extends Store {

  getMetadata() {
    let has = false;
-    for (let node of (this.ast.program.body: Array<Object>)) {
+    for (const node of (this.ast.program.body: Array<Object>)) {
      if (t.isModuleDeclaration(node)) {
        has = true;
        break;
@@ -165,7 +165,7 @@ export default class File extends Store {
      filenameRelative: opts.filename
    });

-    let basenameRelative = path.basename(opts.filenameRelative);
+    const basenameRelative = path.basename(opts.filenameRelative);

    defaults(opts, {
      sourceFileName:   basenameRelative,
@@ -180,13 +180,13 @@ export default class File extends Store {
      return;
    }

-    let plugins: Array<[PluginPass, Object]> = opts.plugins.concat(INTERNAL_PLUGINS);
-    let currentPluginVisitors = [];
-    let currentPluginPasses = [];
+    const plugins: Array<[PluginPass, Object]> = opts.plugins.concat(INTERNAL_PLUGINS);
+    const currentPluginVisitors = [];
+    const currentPluginPasses = [];

    // init plugins!
-    for (let ref of plugins) {
-      let [plugin, pluginOpts] = ref; // todo: fix - can't embed in loop head because of flow bug
+    for (const ref of plugins) {
+      const [plugin, pluginOpts] = ref; // todo: fix - can't embed in loop head because of flow bug

      currentPluginVisitors.push(plugin.visitor);
      currentPluginPasses.push(new PluginPass(this, plugin, pluginOpts));
@@ -201,7 +201,7 @@ export default class File extends Store {
  }

  getModuleName(): ?string {
-    let opts = this.opts;
+    const opts = this.opts;
    if (!opts.moduleIds) {
      return null;
    }
@@ -224,7 +224,7 @@ export default class File extends Store {

    if (opts.sourceRoot != null) {
      // remove sourceRoot from filename
-      let sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
+      const sourceRootRegEx = new RegExp("^" + opts.sourceRoot + "\/?");
      filenameRelative = filenameRelative.replace(sourceRootRegEx, "");
    }

@@ -245,20 +245,20 @@ export default class File extends Store {
  }

  resolveModuleSource(source: string): string {
-    let resolveModuleSource = this.opts.resolveModuleSource;
+    const resolveModuleSource = this.opts.resolveModuleSource;
    if (resolveModuleSource) source = resolveModuleSource(source, this.opts.filename);
    return source;
  }

  addImport(source: string, imported: string, name?: string = imported): Object {
-    let alias = `${source}:${imported}`;
+    const alias = `${source}:${imported}`;
    let id = this.dynamicImportIds[alias];

    if (!id) {
      source = this.resolveModuleSource(source);
      id = this.dynamicImportIds[alias] = this.scope.generateUidIdentifier(name);

-      let specifiers = [];
+      const specifiers = [];

      if (imported === "*") {
        specifiers.push(t.importNamespaceSpecifier(id));
@@ -268,7 +268,7 @@ export default class File extends Store {
        specifiers.push(t.importSpecifier(id, t.identifier(imported)));
      }

-      let declar = t.importDeclaration(specifiers, t.stringLiteral(source));
+      const declar = t.importDeclaration(specifiers, t.stringLiteral(source));
      declar._blockHoist = 3;

      this.path.unshiftContainer("body", declar);
@@ -278,7 +278,7 @@ export default class File extends Store {
  }

  addHelper(name: string): Object {
-    let declar = this.declarations[name];
+    const declar = this.declarations[name];
    if (declar) return declar;

    if (!this.usedHelpers[name]) {
@@ -286,17 +286,17 @@ export default class File extends Store {
      this.usedHelpers[name] = true;
    }

-    let generator = this.get("helperGenerator");
-    let runtime   = this.get("helpersNamespace");
+    const generator = this.get("helperGenerator");
+    const runtime   = this.get("helpersNamespace");
    if (generator) {
-      let res = generator(name);
+      const res = generator(name);
      if (res) return res;
    } else if (runtime) {
      return t.memberExpression(runtime, t.identifier(name));
    }

-    let ref = getHelper(name);
-    let uid = this.declarations[name] = this.scope.generateUidIdentifier(name);
+    const ref = getHelper(name);
+    const uid = this.declarations[name] = this.scope.generateUidIdentifier(name);

    if (t.isFunctionExpression(ref) && !ref.id) {
      ref.body._compact = true;
@@ -323,18 +323,18 @@ export default class File extends Store {
  ): Object {
    // Generate a unique name based on the string literals so we dedupe
    // identical strings used in the program.
-    let stringIds = raw.elements.map(function(string) {
+    const stringIds = raw.elements.map(function(string) {
      return string.value;
    });
-    let name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;
+    const name = `${helperName}_${raw.elements.length}_${stringIds.join(",")}`;

-    let declar = this.declarations[name];
+    const declar = this.declarations[name];
    if (declar) return declar;

-    let uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");
+    const uid = this.declarations[name] = this.scope.generateUidIdentifier("templateObject");

-    let helperId = this.addHelper(helperName);
-    let init = t.callExpression(helperId, [strings, raw]);
+    const helperId = this.addHelper(helperName);
+    const init = t.callExpression(helperId, [strings, raw]);
    init._compact = true;
    this.scope.push({
      id: uid,
@@ -345,9 +345,9 @@ export default class File extends Store {
  }

  buildCodeFrameError(node: Object, msg: string, Error: typeof Error = SyntaxError): Error {
-    let loc = node && (node.loc || node._loc);
+    const loc = node && (node.loc || node._loc);

-    let err = new Error(msg);
+    const err = new Error(msg);

    if (loc) {
      err.loc = loc.start;
@@ -367,13 +367,13 @@ export default class File extends Store {
  }

  mergeSourceMap(map: Object) {
-    let inputMap = this.opts.inputSourceMap;
+    const inputMap = this.opts.inputSourceMap;

    if (inputMap) {
-      let inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
-      let outputMapConsumer  = new sourceMap.SourceMapConsumer(map);
+      const inputMapConsumer   = new sourceMap.SourceMapConsumer(inputMap);
+      const outputMapConsumer  = new sourceMap.SourceMapConsumer(map);

-      let mergedGenerator = new sourceMap.SourceMapGenerator({
+      const mergedGenerator = new sourceMap.SourceMapGenerator({
        file: inputMapConsumer.file,
        sourceRoot: inputMapConsumer.sourceRoot
      });
@@ -402,7 +402,7 @@ export default class File extends Store {
        }
      });

-      let mergedMap = mergedGenerator.toJSON();
+      const mergedMap = mergedGenerator.toJSON();
      inputMap.mappings = mergedMap.mappings;
      return inputMap;
    } else {
@@ -419,8 +419,8 @@ export default class File extends Store {

      if (parserOpts.parser) {
        if (typeof parserOpts.parser === "string") {
-          let dirname = path.dirname(this.opts.filename) || process.cwd();
-          let parser = resolve(parserOpts.parser, dirname);
+          const dirname = path.dirname(this.opts.filename) || process.cwd();
+          const parser = resolve(parserOpts.parser, dirname);
          if (parser) {
            parseCode = require(parser).parse;
          } else {
@@ -439,7 +439,7 @@ export default class File extends Store {
    }

    this.log.debug("Parse start");
-    let ast = parseCode(code, parserOpts || this.parserOpts);
+    const ast = parseCode(code, parserOpts || this.parserOpts);
    this.log.debug("Parse stop");
    return ast;
  }
@@ -472,7 +472,7 @@ export default class File extends Store {
      this.log.debug("Start transform traverse");

      // merge all plugin visitors into a single visitor
-      let visitor = traverse.visitors.merge(this.pluginVisitors[i], pluginPasses, this.opts.wrapPluginVisitorMethod);
+      const visitor = traverse.visitors.merge(this.pluginVisitors[i], pluginPasses, this.opts.wrapPluginVisitorMethod);
      traverse(this.ast, visitor, this.scope);

      this.log.debug("End transform traverse");
@@ -500,7 +500,7 @@ export default class File extends Store {

      let message = err.message = `${this.opts.filename}: ${err.message}`;

-      let loc = err.loc;
+      const loc = err.loc;
      if (loc) {
        err.codeFrame = codeFrame(code, loc.line, loc.column + 1, this.opts);
        message += "\n" + err.codeFrame;
@@ -513,7 +513,7 @@ export default class File extends Store {
      }

      if (err.stack) {
-        let newStack = err.stack.replace(err.message, message);
+        const newStack = err.stack.replace(err.message, message);
        err.stack = newStack;
      }

@@ -529,28 +529,28 @@ export default class File extends Store {

  parseCode() {
    this.parseShebang();
-    let ast = this.parse(this.code);
+    const ast = this.parse(this.code);
    this.addAst(ast);
  }

  shouldIgnore() {
-    let opts = this.opts;
+    const opts = this.opts;
    return util.shouldIgnore(opts.filename, opts.ignore, opts.only);
  }

  call(key: "pre" | "post", pluginPasses: Array<PluginPass>) {
-    for (let pass of pluginPasses) {
-      let plugin = pass.plugin;
-      let fn = plugin[key];
+    for (const pass of pluginPasses) {
+      const plugin = pass.plugin;
+      const fn = plugin[key];
      if (fn) fn.call(pass, this);
    }
  }

  parseInputSourceMap(code: string): string {
-    let opts = this.opts;
+    const opts = this.opts;

    if (opts.inputSourceMap !== false) {
-      let inputMap = convertSourceMap.fromSource(code);
+      const inputMap = convertSourceMap.fromSource(code);
      if (inputMap) {
        opts.inputSourceMap = inputMap.toObject();
        code = convertSourceMap.removeComments(code);
@@ -561,7 +561,7 @@ export default class File extends Store {
  }

  parseShebang() {
-    let shebangMatch = shebangRegex.exec(this.code);
+    const shebangMatch = shebangRegex.exec(this.code);
    if (shebangMatch) {
      this.shebang = shebangMatch[0];
      this.code = this.code.replace(shebangRegex, "");
@@ -569,7 +569,7 @@ export default class File extends Store {
  }

  makeResult({ code, map, ast, ignored }: BabelFileResult): BabelFileResult {
-    let result = {
+    const result = {
      metadata: null,
      options:  this.opts,
      ignored:  !!ignored,
@@ -594,10 +594,10 @@ export default class File extends Store {
  }

  generate(): BabelFileResult {
-    let opts = this.opts;
-    let ast  = this.ast;
+    const opts = this.opts;
+    const ast  = this.ast;

-    let result: BabelFileResult = { ast };
+    const result: BabelFileResult = { ast };
    if (!opts.code) return this.makeResult(result);

    let gen = generate;
@@ -605,8 +605,8 @@ export default class File extends Store {
      gen = opts.generatorOpts.generator;

      if (typeof gen === "string") {
-        let dirname = path.dirname(this.opts.filename) || process.cwd();
-        let generator = resolve(gen, dirname);
+        const dirname = path.dirname(this.opts.filename) || process.cwd();
+        const generator = resolve(gen, dirname);
        if (generator) {
          gen = require(generator).print;
        } else {
@@ -617,7 +617,7 @@ export default class File extends Store {

    this.log.debug("Generation start");

-    let _result = gen(ast, opts.generatorOpts ? Object.assign(opts, opts.generatorOpts) : opts, this.code);
+    const _result = gen(ast, opts.generatorOpts ? Object.assign(opts, opts.generatorOpts) : opts, this.code);
    result.code = _result.code;
    result.map  = _result.map;

--- a/packages/babel-core/src/transformation/file/logger.js
+++ b/packages/babel-core/src/transformation/file/logger.js
@@ -1,10 +1,10 @@
 import type File from "./index";
 import buildDebug from "debug/node";

-let verboseDebug = buildDebug("babel:verbose");
-let generalDebug = buildDebug("babel");
+const verboseDebug = buildDebug("babel:verbose");
+const generalDebug = buildDebug("babel");

-let seenDeprecatedMessages = [];
+const seenDeprecatedMessages = [];

 export default class Logger {
  constructor(file: File, filename: string) {
--- a/packages/babel-core/src/transformation/file/metadata.js
+++ b/packages/babel-core/src/transformation/file/metadata.js
@@ -1,28 +1,28 @@
 import * as t from "babel-types";

-export let ModuleDeclaration = {
+export const ModuleDeclaration = {
  enter(path, file) {
-    let { node } = path;
+    const { node } = path;
    if (node.source) {
      node.source.value = file.resolveModuleSource(node.source.value);
    }
  }
 };

-export let ImportDeclaration = {
+export const ImportDeclaration = {
  exit(path, file) {
-    let { node } = path;
+    const { node } = path;

-    let specifiers = [];
-    let imported = [];
+    const specifiers = [];
+    const imported = [];
    file.metadata.modules.imports.push({
      source: node.source.value,
      imported,
      specifiers
    });

-    for (let specifier of (path.get("specifiers"): Array<Object>)) {
-      let local = specifier.node.local.name;
+    for (const specifier of (path.get("specifiers"): Array<Object>)) {
+      const local = specifier.node.local.name;

      if (specifier.isImportDefaultSpecifier()) {
        imported.push("default");
@@ -34,7 +34,7 @@ export let ImportDeclaration = {
      }

      if (specifier.isImportSpecifier()) {
-        let importedName = specifier.node.imported.name;
+        const importedName = specifier.node.imported.name;
        imported.push(importedName);
        specifiers.push({
          kind: "named",
@@ -55,18 +55,18 @@ export let ImportDeclaration = {
 };

 export function ExportDeclaration(path, file) {
-  let { node } = path;
+  const { node } = path;

-  let source = node.source ? node.source.value : null;
-  let exports = file.metadata.modules.exports;
+  const source = node.source ? node.source.value : null;
+  const exports = file.metadata.modules.exports;

  // export function foo() {}
  // export let foo = "bar";
-  let declar = path.get("declaration");
+  const declar = path.get("declaration");
  if (declar.isStatement()) {
-    let bindings = declar.getBindingIdentifiers();
+    const bindings = declar.getBindingIdentifiers();

-    for (let name in bindings) {
+    for (const name in bindings) {
      exports.exported.push(name);
      exports.specifiers.push({
        kind: "local",
@@ -77,8 +77,8 @@ export function ExportDeclaration(path, file) {
  }

  if (path.isExportNamedDeclaration() && node.specifiers) {
-    for (let specifier of (node.specifiers: Array<Object>)) {
-      let exported = specifier.exported.name;
+    for (const specifier of (node.specifiers: Array<Object>)) {
+      const exported = specifier.exported.name;
      exports.exported.push(exported);

      // export foo from "bar";
@@ -100,7 +100,7 @@ export function ExportDeclaration(path, file) {
        });
      }

-      let local = specifier.local;
+      const local = specifier.local;
      if (!local) continue;

      // export { foo } from "bar";
--- a/packages/babel-core/src/transformation/file/options/build-config-chain.js
+++ b/packages/babel-core/src/transformation/file/options/build-config-chain.js
@@ -6,15 +6,15 @@ import isAbsolute from "path-is-absolute";
 import path from "path";
 import fs from "fs";

-let existsCache = {};
-let jsonCache   = {};
+const existsCache = {};
+const jsonCache   = {};

 const BABELIGNORE_FILENAME = ".babelignore";
 const BABELRC_FILENAME     = ".babelrc";
 const PACKAGE_FILENAME     = "package.json";

 function exists(filename) {
-  let cached = existsCache[filename];
+  const cached = existsCache[filename];
  if (cached == null) {
    return existsCache[filename] = fs.existsSync(filename);
  } else {
@@ -23,8 +23,8 @@ function exists(filename) {
 }

 export default function buildConfigChain(opts: Object = {}, log?: Logger) {
-  let filename = opts.filename;
-  let builder = new ConfigChainBuilder(log);
+  const filename = opts.filename;
+  const builder = new ConfigChainBuilder(log);

  // resolve all .babelrc files
  if (opts.babelrc !== false) {
@@ -59,20 +59,20 @@ class ConfigChainBuilder {

    while (loc !== (loc = path.dirname(loc))) {
      if (!foundConfig) {
-        let configLoc = path.join(loc, BABELRC_FILENAME);
+        const configLoc = path.join(loc, BABELRC_FILENAME);
        if (exists(configLoc)) {
          this.addConfig(configLoc);
          foundConfig = true;
        }

-        let pkgLoc = path.join(loc, PACKAGE_FILENAME);
+        const pkgLoc = path.join(loc, PACKAGE_FILENAME);
        if (!foundConfig && exists(pkgLoc)) {
          foundConfig = this.addConfig(pkgLoc, "babel", JSON);
        }
      }

      if (!foundIgnore) {
-        let ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
+        const ignoreLoc = path.join(loc, BABELIGNORE_FILENAME);
        if (exists(ignoreLoc)) {
          this.addIgnoreConfig(ignoreLoc);
          foundIgnore = true;
@@ -84,7 +84,7 @@ class ConfigChainBuilder {
  }

  addIgnoreConfig(loc) {
-    let file  = fs.readFileSync(loc, "utf8");
+    const file  = fs.readFileSync(loc, "utf8");
    let lines = file.split("\n");

    lines = lines
@@ -107,7 +107,7 @@ class ConfigChainBuilder {

    this.resolvedConfigs.push(loc);

-    let content = fs.readFileSync(loc, "utf8");
+    const content = fs.readFileSync(loc, "utf8");
    let options;

    try {
@@ -144,7 +144,7 @@ class ConfigChainBuilder {

    // add extends clause
    if (options.extends) {
-      let extendsLoc = resolve(options.extends, dirname);
+      const extendsLoc = resolve(options.extends, dirname);
      if (extendsLoc) {
        this.addConfig(extendsLoc);
      } else {
@@ -162,7 +162,7 @@ class ConfigChainBuilder {

    // env
    let envOpts;
-    let envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
+    const envKey = process.env.BABEL_ENV || process.env.NODE_ENV || "development";
    if (options.env) {
      envOpts = options.env[envKey];
      delete options.env;
--- a/packages/babel-core/src/transformation/file/options/index.js
+++ b/packages/babel-core/src/transformation/file/options/index.js
@@ -4,7 +4,7 @@ import config from "./config";
 export { config };

 export function normaliseOptions(options: Object = {}): Object {
-  for (let key in options) {
+  for (const key in options) {
    let val = options[key];
    if (val == null) continue;

@@ -12,7 +12,7 @@ export function normaliseOptions(options: Object = {}): Object {
    if (opt && opt.alias) opt = config[opt.alias];
    if (!opt) continue;

-    let parser = parsers[opt.type];
+    const parser = parsers[opt.type];
    if (parser) val = parser(val);

    options[key] = val;
--- a/packages/babel-core/src/transformation/file/options/option-manager.js
+++ b/packages/babel-core/src/transformation/file/options/option-manager.js
@@ -53,7 +53,7 @@ export default class OptionManager {
  }>;

  static memoisePluginContainer(fn, loc, i, alias) {
-    for (let cache of (OptionManager.memoisedPlugins: Array<Object>)) {
+    for (const cache of (OptionManager.memoisedPlugins: Array<Object>)) {
      if (cache.container === fn) return cache.plugin;
    }

@@ -66,7 +66,7 @@ export default class OptionManager {
    }

    if (typeof obj === "object") {
-      let plugin = new Plugin(obj, alias);
+      const plugin = new Plugin(obj, alias);
      OptionManager.memoisedPlugins.push({
        container: fn,
        plugin: plugin
@@ -78,10 +78,10 @@ export default class OptionManager {
  }

  static createBareOptions() {
-    let opts = {};
+    const opts = {};

-    for (let key in config) {
-      let opt = config[key];
+    for (const key in config) {
+      const opt = config[key];
      opts[key] = clone(opt.default);
    }

@@ -120,11 +120,11 @@ export default class OptionManager {
        plugin = val;
      }

-      let alias = typeof plugin === "string" ? plugin : `${loc}$${i}`;
+      const alias = typeof plugin === "string" ? plugin : `${loc}$${i}`;

      // allow plugins to be specified as strings
      if (typeof plugin === "string") {
-        let pluginLoc = resolvePlugin(plugin, dirname);
+        const pluginLoc = resolvePlugin(plugin, dirname);
        if (pluginLoc) {
          plugin = require(pluginLoc);
        } else {
@@ -164,7 +164,7 @@ export default class OptionManager {
    }

    //
-    let opts = cloneDeepWith(rawOpts, (val) => {
+    const opts = cloneDeepWith(rawOpts, (val) => {
      if (val instanceof Plugin) {
        return val;
      }
@@ -174,16 +174,16 @@ export default class OptionManager {
    dirname = dirname || process.cwd();
    loc = loc || alias;

-    for (let key in opts) {
-      let option = config[key];
+    for (const key in opts) {
+      const option = config[key];

      // check for an unknown option
      if (!option && this.log) {
        if (removed[key]) {
          this.log.error(`Using removed Babel 5 option: ${alias}.${key} - ${removed[key].message}`, ReferenceError);
        } else {
-          let unknownOptErr = `Unknown option: ${alias}.${key}. Check out http://babeljs.io/docs/usage/options/ for more information about options.`;
-          let presetConfigErr = "A common cause of this error is the presence of a configuration options object without the corresponding preset name. Example:\n\nInvalid:\n  `{ presets: [{option: value}] }`\nValid:\n  `{ presets: [['presetName', {option: value}]] }`\n\nFor more detailed information on preset configuration, please see http://babeljs.io/docs/plugins/#pluginpresets-options.";
+          const unknownOptErr = `Unknown option: ${alias}.${key}. Check out http://babeljs.io/docs/usage/options/ for more information about options.`;
+          const presetConfigErr = "A common cause of this error is the presence of a configuration options object without the corresponding preset name. Example:\n\nInvalid:\n  `{ presets: [{option: value}] }`\nValid:\n  `{ presets: [['presetName', {option: value}]] }`\n\nFor more detailed information on preset configuration, please see http://babeljs.io/docs/plugins/#pluginpresets-options.";

          this.log.error(`${unknownOptErr}\n\n${presetConfigErr}`, ReferenceError);
        }
@@ -316,11 +316,11 @@ export default class OptionManager {
  }

  normaliseOptions() {
-    let opts = this.options;
+    const opts = this.options;

-    for (let key in config) {
-      let option = config[key];
-      let val    = opts[key];
+    for (const key in config) {
+      const option = config[key];
+      const val    = opts[key];

      // optional
      if (!val && option.optional) continue;
@@ -335,7 +335,7 @@ export default class OptionManager {
  }

  init(opts: Object = {}): Object {
-    for (let config of buildConfigChain(opts, this.log)) {
+    for (const config of buildConfigChain(opts, this.log)) {
      this.mergeOptions(config);
    }

--- a/packages/babel-core/src/transformation/file/options/parsers.js
+++ b/packages/babel-core/src/transformation/file/options/parsers.js
@@ -1,7 +1,7 @@
 import slash from "slash";
 import * as util from "../../../util";

-export let filename = slash;
+export const filename = slash;

 export function boolean(val: any): boolean {
  return !!val;
--- a/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
+++ b/packages/babel-core/src/transformation/internal-plugins/block-hoist.js
@@ -20,7 +20,7 @@ export default new Plugin({
      exit({ node }) {
        let hasChange = false;
        for (let i = 0; i < node.body.length; i++) {
-          let bodyNode = node.body[i];
+          const bodyNode = node.body[i];
          if (bodyNode && bodyNode._blockHoist != null) {
            hasChange = true;
            break;
--- a/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
+++ b/packages/babel-core/src/transformation/internal-plugins/shadow-functions.js
@@ -41,10 +41,10 @@ function shouldShadow(path, shadowPath) {

 function remap(path, key) {
  // ensure that we're shadowed
-  let shadowPath = path.inShadow(key);
+  const shadowPath = path.inShadow(key);
  if (!shouldShadow(path, shadowPath)) return;

-  let shadowFunction = path.node._shadowedFunctionLiteral;
+  const shadowFunction = path.node._shadowedFunctionLiteral;

  let currentFunction;
  let passedShadowFunction = false;
@@ -91,15 +91,15 @@ function remap(path, key) {
  // binding since arrow function syntax already does that.
  if (!passedShadowFunction) return;

-  let cached = fnPath.getData(key);
+  const cached = fnPath.getData(key);
  if (cached) return path.replaceWith(cached);

-  let id   = path.scope.generateUidIdentifier(key);
+  const id   = path.scope.generateUidIdentifier(key);

  fnPath.setData(key, id);

-  let classPath = fnPath.findParent((p) => p.isClass());
-  let hasSuperClass = !!(classPath && classPath.node && classPath.node.superClass);
+  const classPath = fnPath.findParent((p) => p.isClass());
+  const hasSuperClass = !!(classPath && classPath.node && classPath.node.superClass);

  if (key === "this" && fnPath.isMethod({kind: "constructor"}) && hasSuperClass) {
    fnPath.scope.push({ id });
--- a/packages/babel-core/src/transformation/pipeline.js
+++ b/packages/babel-core/src/transformation/pipeline.js
@@ -11,7 +11,7 @@ export default class Pipeline {
  }

  pretransform(code: string, opts?: Object): BabelFileResult {
-    let file = new File(opts, this);
+    const file = new File(opts, this);
    return file.wrap(code, function () {
      file.addCode(code);
      file.parseCode(code);
@@ -20,7 +20,7 @@ export default class Pipeline {
  }

  transform(code: string, opts?: Object): BabelFileResult {
-    let file = new File(opts, this);
+    const file = new File(opts, this);
    return file.wrap(code, function () {
      file.addCode(code);
      file.parseCode(code);
@@ -40,7 +40,7 @@ export default class Pipeline {
  transformFromAst(ast: Object, code: string, opts: Object): BabelFileResult {
    ast = normalizeAst(ast);

-    let file = new File(opts, this);
+    const file = new File(opts, this);
    return file.wrap(code, function () {
      file.addCode(code);
      file.addAst(ast);
--- a/packages/babel-core/src/transformation/plugin.js
+++ b/packages/babel-core/src/transformation/plugin.js
@@ -31,7 +31,7 @@ export default class Plugin extends Store {
  visitor: Object;

  take(key) {
-    let val = this.raw[key];
+    const val = this.raw[key];
    delete this.raw[key];
    return val;
  }
@@ -40,13 +40,13 @@ export default class Plugin extends Store {
    if (!target[key]) return this[key];
    if (!this[key]) return target[key];

-    let fns: Array<?Function> = [target[key], this[key]];
+    const fns: Array<?Function> = [target[key], this[key]];

    return function (...args) {
      let val;
-      for (let fn of fns) {
+      for (const fn of fns) {
        if (fn) {
-          let ret = fn.apply(this, args);
+          const ret = fn.apply(this, args);
          if (ret != null) val = ret;
        }
      }
@@ -77,13 +77,13 @@ export default class Plugin extends Store {

    this.maybeInherit(loc);

-    for (let key in this.raw) {
+    for (const key in this.raw) {
      throw new Error(messages.get("pluginInvalidProperty", loc, i, key));
    }
  }

  normaliseVisitor(visitor: Object): Object {
-    for (let key of GLOBAL_VISITOR_PROPS) {
+    for (const key of GLOBAL_VISITOR_PROPS) {
      if (visitor[key]) {
        throw new Error("Plugins aren't allowed to specify catch-all enter/exit handlers. Please target individual nodes.");
      }
--- a/packages/babel-core/src/util.js
+++ b/packages/babel-core/src/util.js
@@ -15,8 +15,8 @@ export { inherits, inspect } from "util";
 */

 export function canCompile(filename: string, altExts?: Array<string>): boolean {
-  let exts = altExts || canCompile.EXTENSIONS;
-  let ext = path.extname(filename);
+  const exts = altExts || canCompile.EXTENSIONS;
+  const ext = path.extname(filename);
  return includes(exts, ext);
 }

@@ -63,7 +63,7 @@ export function regexify(val: any): RegExp {
    if (startsWith(val, "./") || startsWith(val, "*/")) val = val.slice(2);
    if (startsWith(val, "**/")) val = val.slice(3);

-    let regex = minimatch.makeRe(val, { nocase: true });
+    const regex = minimatch.makeRe(val, { nocase: true });
    return new RegExp(regex.source.slice(1, -1), "i");
  }

@@ -119,12 +119,12 @@ export function shouldIgnore(
  filename = filename.replace(/\\/g, "/");

  if (only) {
-    for (let pattern of only) {
+    for (const pattern of only) {
      if (_shouldIgnore(pattern, filename)) return false;
    }
    return true;
  } else if (ignore.length) {
-    for (let pattern of ignore) {
+    for (const pattern of ignore) {
      if (_shouldIgnore(pattern, filename)) return true;
    }
  }
--- a/packages/babel-core/test/api.js
+++ b/packages/babel-core/test/api.js
@@ -1,9 +1,9 @@
-let babel                = require("../lib/api/node");
-let buildExternalHelpers = require("../lib/tools/build-external-helpers");
-let sourceMap            = require("source-map");
-let assert               = require("assert");
-let Plugin               = require("../lib/transformation/plugin");
-let generator            = require("babel-generator").default;
+const babel                = require("../lib/api/node");
+const buildExternalHelpers = require("../lib/tools/build-external-helpers");
+const sourceMap            = require("source-map");
+const assert               = require("assert");
+const Plugin               = require("../lib/transformation/plugin");
+const generator            = require("babel-generator").default;

 function assertIgnored(result) {
  assert.ok(result.ignored);
@@ -23,7 +23,7 @@ function transformAsync(code, opts) {
 }

 describe("parser and generator options", function() {
-  let recast = {
+  const recast = {
    parse: function(code, opts) {
      return opts.parser.parse(code);
    },
@@ -46,13 +46,13 @@ describe("parser and generator options", function() {
  }

  it("options", function() {
-    let string = "original;";
+    const string = "original;";
    assert.deepEqual(newTransform(string).ast, babel.transform(string).ast);
    assert.equal(newTransform(string).code, string);
  });

  it("experimental syntax", function() {
-    let experimental = "var a: number = 1;";
+    const experimental = "var a: number = 1;";

    assert.deepEqual(newTransform(experimental).ast, babel.transform(experimental, {
      parserOpts: {
@@ -82,7 +82,7 @@ describe("parser and generator options", function() {
  });

  it("other options", function() {
-    let experimental = "if (true) {\n  import a from 'a';\n}";
+    const experimental = "if (true) {\n  import a from 'a';\n}";

    assert.notEqual(newTransform(experimental).ast, babel.transform(experimental, {
      parserOpts: {
@@ -199,7 +199,7 @@ describe("api", function () {
              new Plugin({
                visitor: {
                  Function: function(path) {
-                    let alias = path.scope.getProgramParent().path.get("body")[0].node;
+                    const alias = path.scope.getProgramParent().path.get("body")[0].node;
                    if (!babel.types.isTypeAlias(alias)) return;

                    // In case of `passPerPreset` being `false`, the
@@ -264,7 +264,7 @@ describe("api", function () {
  });

  it("source map merging", function () {
-    let result = babel.transform([
+    const result = babel.transform([
      "function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError(\"Cannot call a class as a function\"); } }",
      "",
      "let Foo = function Foo() {",
@@ -288,7 +288,7 @@ describe("api", function () {
      "};"
    ].join("\n"), result.code);

-    let consumer = new sourceMap.SourceMapConsumer(result.map);
+    const consumer = new sourceMap.SourceMapConsumer(result.map);

    assert.deepEqual(consumer.originalPositionFor({
      line: 7,
@@ -318,7 +318,7 @@ describe("api", function () {
      auxiliaryCommentBefore: "before",
      auxiliaryCommentAfter: "after",
      plugins: [function (babel) {
-        let t = babel.types;
+        const t = babel.types;
        return {
          visitor: {
            Program: function (path) {
@@ -539,8 +539,8 @@ describe("api", function () {
  });

  describe("env option", function () {
-    let oldBabelEnv = process.env.BABEL_ENV;
-    let oldNodeEnv = process.env.NODE_ENV;
+    const oldBabelEnv = process.env.BABEL_ENV;
+    const oldNodeEnv = process.env.NODE_ENV;

    setup(function () {
      // Tests need to run with the default and specific values for these. They
@@ -555,7 +555,7 @@ describe("api", function () {
    });

    it("default", function () {
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
        env: {
          development: { code: false }
        }
@@ -566,7 +566,7 @@ describe("api", function () {

    it("BABEL_ENV", function () {
      process.env.BABEL_ENV = "foo";
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
        env: {
          foo: { code: false }
        }
@@ -576,7 +576,7 @@ describe("api", function () {

    it("NODE_ENV", function () {
      process.env.NODE_ENV = "foo";
-      let result = babel.transform("foo;", {
+      const result = babel.transform("foo;", {
        env: {
          foo: { code: false }
        }
@@ -586,8 +586,8 @@ describe("api", function () {
  });

  it("resolveModuleSource option", function () {
-    let actual = "import foo from \"foo-import-default\";\nimport \"foo-import-bare\";\nexport { foo } from \"foo-export-named\";";
-    let expected = "import foo from \"resolved/foo-import-default\";\nimport \"resolved/foo-import-bare\";\nexport { foo } from \"resolved/foo-export-named\";";
+    const actual = "import foo from \"foo-import-default\";\nimport \"foo-import-bare\";\nexport { foo } from \"foo-export-named\";";
+    const expected = "import foo from \"resolved/foo-import-default\";\nimport \"resolved/foo-import-bare\";\nexport { foo } from \"resolved/foo-export-named\";";

    return transformAsync(actual, {
      resolveModuleSource: function (originalSource) {
@@ -600,25 +600,25 @@ describe("api", function () {

  describe("buildExternalHelpers", function () {
    it("all", function () {
-      let script = buildExternalHelpers();
+      const script = buildExternalHelpers();
      assert.ok(script.indexOf("classCallCheck") >= -1);
      assert.ok(script.indexOf("inherits") >= 0);
    });

    it("whitelist", function () {
-      let script = buildExternalHelpers(["inherits"]);
+      const script = buildExternalHelpers(["inherits"]);
      assert.ok(script.indexOf("classCallCheck") === -1);
      assert.ok(script.indexOf("inherits") >= 0);
    });

    it("empty whitelist", function () {
-      let script = buildExternalHelpers([]);
+      const script = buildExternalHelpers([]);
      assert.ok(script.indexOf("classCallCheck") === -1);
      assert.ok(script.indexOf("inherits") === -1);
    });

    it("underscored", function () {
-      let script = buildExternalHelpers(["typeof"]);
+      const script = buildExternalHelpers(["typeof"]);
      assert.ok(script.indexOf("typeof") >= 0);
    });
  });
--- a/packages/babel-core/test/browserify.js
+++ b/packages/babel-core/test/browserify.js
@@ -1,11 +1,11 @@
-let browserify = require("browserify");
-let assert = require("assert");
-let path = require("path");
-let vm = require("vm");
+const browserify = require("browserify");
+const assert = require("assert");
+const path = require("path");
+const vm = require("vm");

 describe("browserify", function() {
  it("babel/register may be used without breaking browserify", function(done) {
-    let bundler = browserify(path.join(__dirname, "fixtures/browserify/register.js"));
+    const bundler = browserify(path.join(__dirname, "fixtures/browserify/register.js"));

    bundler.bundle(function(err, bundle) {
      if (err) return done(err);
--- a/packages/babel-core/test/config-chain.js
+++ b/packages/babel-core/test/config-chain.js
@@ -1,9 +1,9 @@
-let assert = require("assert");
-let path = require("path");
-let buildConfigChain = require("../lib/transformation/file/options/build-config-chain");
+const assert = require("assert");
+const path = require("path");
+const buildConfigChain = require("../lib/transformation/file/options/build-config-chain");

 function fixture() {
-  let args = [__dirname, "fixtures", "config"];
+  const args = [__dirname, "fixtures", "config"];
  for (let i = 0; i < arguments.length; i ++) {
    args.push(arguments[i]);
  }
@@ -28,11 +28,11 @@ describe("buildConfigChain", function () {
  });

  it("dir1", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("dir1", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: [
@@ -77,11 +77,11 @@ describe("buildConfigChain", function () {
  });

  it("dir2", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("dir2", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: [
@@ -116,11 +116,11 @@ describe("buildConfigChain", function () {
  });

  it("env - base", function () {
-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("env", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: [
@@ -157,11 +157,11 @@ describe("buildConfigChain", function () {
  it("env - foo", function () {
    process.env.NODE_ENV = "foo";

-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("env", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: [
@@ -209,11 +209,11 @@ describe("buildConfigChain", function () {
    process.env.NODE_ENV = "foo"; // overridden
    process.env.NODE_ENV = "bar";

-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("env", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: [
@@ -261,11 +261,11 @@ describe("buildConfigChain", function () {
  it("env - foo", function () {
    process.env.NODE_ENV = "foo";

-    let chain = buildConfigChain({
+    const chain = buildConfigChain({
      filename: fixture("pkg", "src.js")
    });

-    let expected = [
+    const expected = [
      {
        options: {
          plugins: ["pkg-plugin"]
--- a/packages/babel-core/test/evaluation.js
+++ b/packages/babel-core/test/evaluation.js
@@ -1,14 +1,14 @@
-let traverse = require("babel-traverse").default;
-let assert   = require("assert");
-let parse    = require("babylon").parse;
+const traverse = require("babel-traverse").default;
+const assert   = require("assert");
+const parse    = require("babylon").parse;

 describe("evaluation", function () {
  function addTest(code, type, value, notConfident) {
    it(type + ": " + code, function () {
-      let visitor = {};
+      const visitor = {};

      visitor[type] = function (path) {
-        let evaluate = path.evaluate();
+        const evaluate = path.evaluate();
        assert.equal(evaluate.confident, !notConfident);
        assert.deepEqual(evaluate.value, value);
      };
--- a/packages/babel-core/test/get-possible-plugin-names.js
+++ b/packages/babel-core/test/get-possible-plugin-names.js
@@ -1,5 +1,5 @@
-let assert                 = require("assert");
-let getPossiblePluginNames = require("../lib/helpers/get-possible-plugin-names");
+const assert                 = require("assert");
+const getPossiblePluginNames = require("../lib/helpers/get-possible-plugin-names");

 describe("getPossiblePluginNames", function () {
  it("adds the babel-plugin prefix", function() {
--- a/packages/babel-core/test/get-possible-preset-names.js
+++ b/packages/babel-core/test/get-possible-preset-names.js
@@ -1,5 +1,5 @@
-let assert                 = require("assert");
-let getPossiblePresetNames = require("../lib/helpers/get-possible-preset-names");
+const assert                 = require("assert");
+const getPossiblePresetNames = require("../lib/helpers/get-possible-preset-names");

 describe("getPossiblePresetNames", function () {
  it("adds the babel-preset prefix", function() {
--- a/packages/babel-core/test/option-manager.js
+++ b/packages/babel-core/test/option-manager.js
@@ -17,7 +17,7 @@ describe("option-manager", () => {
    it("throws for removed babel 5 options", () => {
      return assert.throws(
        () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
          opt.init({
            "randomOption": true
          });
@@ -29,7 +29,7 @@ describe("option-manager", () => {
    it("throws for removed babel 5 options", () => {
      return assert.throws(
        () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
          opt.init({
            "auxiliaryComment": true,
            "blacklist": true
@@ -42,7 +42,7 @@ describe("option-manager", () => {
    it("throws for resolved but erroring preset", () => {
      return assert.throws(
        () => {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
          opt.init({
            "presets": [path.join(__dirname, "fixtures/option-manager/not-a-preset")]
          });
@@ -54,7 +54,7 @@ describe("option-manager", () => {
    it("throws for invalid preset configuration", function() {
      return assert.throws(
        function () {
-          let opt = new OptionManager(new Logger(null, "unknown"));
+          const opt = new OptionManager(new Logger(null, "unknown"));
          opt.init({
            "presets": [{ option: "value" }]
          });
@@ -67,8 +67,8 @@ describe("option-manager", () => {
  describe("presets", function () {
    function presetTest(name) {
      it(name, function () {
-        let opt = new OptionManager(new Logger(null, "unknown"));
-        let options = opt.init({
+        const opt = new OptionManager(new Logger(null, "unknown"));
+        const options = opt.init({
          "presets": [path.join(__dirname, "fixtures/option-manager/presets", name)]
        });

--- a/packages/babel-core/test/path.js
+++ b/packages/babel-core/test/path.js
@@ -1,12 +1,12 @@
-let transform = require("../lib/api/node").transform;
-let Plugin    = require("../lib/transformation/plugin");
-let chai      = require("chai");
+const transform = require("../lib/api/node").transform;
+const Plugin    = require("../lib/transformation/plugin");
+const chai      = require("chai");

 describe("traversal path", function () {
  it("replaceWithSourceString", function () {
-    let expectCode = "function foo() {}";
+    const expectCode = "function foo() {}";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          FunctionDeclaration: function (path) {
@@ -20,9 +20,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (arrow expression body to block statement body)", function () {
-    let expectCode = "var fn = () => true;";
+    const expectCode = "var fn = () => true;";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ArrowFunctionExpression: function (path) {
@@ -45,9 +45,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (arrow block statement body to expression body)", function () {
-    let expectCode = "var fn = () => { return true; }";
+    const expectCode = "var fn = () => { return true; }";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ArrowFunctionExpression: function (path) {
@@ -64,9 +64,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (for-in left expression to variable declaration)", function () {
-    let expectCode = "for (KEY in right);";
+    const expectCode = "for (KEY in right);";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ForInStatement: function (path) {
@@ -90,9 +90,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (for-in left variable declaration to expression)", function () {
-    let expectCode = "for (var KEY in right);";
+    const expectCode = "for (var KEY in right);";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ForInStatement: function (path) {
@@ -109,9 +109,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (for-loop left expression to variable declaration)", function () {
-    let expectCode = "for (KEY;;);";
+    const expectCode = "for (KEY;;);";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ForStatement: function (path) {
@@ -135,9 +135,9 @@ describe("traversal path", function () {
  });

  it("replaceWith (for-loop left variable declaration to expression)", function () {
-    let expectCode = "for (var KEY;;);";
+    const expectCode = "for (var KEY;;);";

-    let actualCode = transform(expectCode, {
+    const actualCode = transform(expectCode, {
      plugins: [new Plugin({
        visitor: {
          ForStatement: function (path) {
--- a/packages/babel-core/test/resolution.js
+++ b/packages/babel-core/test/resolution.js
@@ -1,14 +1,14 @@
-let assert = require("assert");
-let async = require("async");
-let babel = require("../lib/api/node");
-let fs = require("fs");
-let path = require("path");
+const assert = require("assert");
+const async = require("async");
+const babel = require("../lib/api/node");
+const fs = require("fs");
+const path = require("path");

 // Test that plugins & presets are resolved relative to `filename`.
 describe("addon resolution", function () {
  it("addon resolution", function (done) {
-    let fixtures = {};
-    let paths = {};
+    const fixtures = {};
+    const paths = {};

    paths.fixtures = path.join(
      __dirname,
@@ -33,7 +33,7 @@ describe("addon resolution", function () {
    function fixturesReady (err) {
      if (err) return done(err);

-      let actual = babel.transform(fixtures.actual, {
+      const actual = babel.transform(fixtures.actual, {
        filename: paths.actual,
        plugins: ["addons/plugin"],
        presets: ["addons/preset"],
--- a/packages/babel-core/test/util.js
+++ b/packages/babel-core/test/util.js
@@ -1,6 +1,6 @@
-let assert = require("assert");
-let util   = require("../lib/util");
-let t      = require("babel-types");
+const assert = require("assert");
+const util   = require("../lib/util");
+const t      = require("babel-types");

 describe("util", function () {
  it("canCompile", function () {
@@ -36,7 +36,7 @@ describe("util", function () {
    assert.deepEqual(util.list(["foo", "bar"]), ["foo", "bar"]);
    assert.deepEqual(util.list(/foo/), [/foo/]);

-    let date = new Date;
+    const date = new Date;
    assert.deepEqual(util.list(date), [date]);
  });

@@ -84,8 +84,8 @@ describe("util", function () {
  });

  it("shouldIgnore", function () {
-    let reIgnore = /\-reIgnore\.js/;
-    let fnIgnore = function (src) {
+    const reIgnore = /\-reIgnore\.js/;
+    const fnIgnore = function (src) {
      if (src.indexOf("fnIgnore") > 0) {
        return true;
      }