add access to helpers used by the current file and allow a helper whitelist to be passed to buildHelpers - closes #898

src/babel/build-external-helpers.js

@@ -3,7 +3,7 @@ var generator    = require("./generation");
 var util         = require("./util");
 var t            = require("./types");
 
-module.exports = function () {
+module.exports = function (whitelist) {
   var namespace = t.identifier("babelHelpers");
 
   var body      = [];
@@ -17,7 +17,7 @@ module.exports = function () {
     )
   ]));
 
-  buildHelpers(body, namespace);
+  buildHelpers(body, namespace, whitelist);
 
   return generator(tree).code;
 };

src/babel/build-helpers.js

@@ -3,9 +3,11 @@ var util = require("./util");
 var each = require("lodash/collection/each");
 var t    = require("./types");
 
-module.exports = function (body, namespace) {
+module.exports = function (body, namespace, whitelist = []) {
   each(File.helpers, function (name) {
     var key = t.identifier(t.toIdentifier(name));
+    if (whitelist.length && whitelist.indexOf(key) >= 0) return;
+
     body.push(t.expressionStatement(
       t.assignmentExpression("=", t.memberExpression(namespace, key), util.template(name))
     ));

src/babel/transformation/file.js

@@ -22,6 +22,7 @@ function File(opts) {
   this.dynamicImported          = [];
   this.dynamicImports           = [];
 
+  this.usedHelpers = {};
   this.dynamicData = {};
   this.data        = {};
 
@@ -85,6 +86,7 @@ File.validOptions = [
   "externalHelpers",
   "auxiliaryComment",
   "compact",
+  "returnUsedHelpers",
 
   "resolveModuleSource",
   "moduleId",
@@ -111,6 +113,7 @@ File.prototype.normalizeOptions = function (opts) {
   defaults(opts, {
     keepModuleIdExtensions: false,
     resolveModuleSource:    null,
+    returnUsedHelpers:      false,
     externalHelpers:        false,
     auxilaryComment:        "",
     experimental:           false,
@@ -331,6 +334,8 @@ File.prototype.addHelper = function (name) {
   var declar = program._declarations && program._declarations[name];
   if (declar) return declar.id;
 
+  this.usedHelpers[name] = true;
+
   var runtime = this.get("helpersNamespace");
   if (runtime) {
     name = t.identifier(t.toIdentifier(name));
@@ -448,6 +453,10 @@ File.prototype.generate = function () {
     ast: null
   };
 
+  if (this.opts.returnUsedHelpers) {
+    result.usedHelpers = Object.keys(this.usedHelpers);
+  }
+
   if (opts.ast) result.ast = ast;
   if (!opts.code) return result;