handle comments and use strict directives better - fixes #1030

src/babel/transformation/transformers/internal/strict.js

@@ -2,6 +2,17 @@ import * as t from "../../../types";
 
 export function Program(program, parent, scope, file) {
   if (file.transformers.strict.canRun()) {
-    program.body.unshift(t.expressionStatement(t.literal("use strict")));
+    var directive = file.get("existingStrictDirective");
+
+    if (!directive) {
+      directive = t.expressionStatement(t.literal("use strict"));
+      var first = program.body[0];
+      if (first) {
+        directive.leadingComments = first.leadingComments;
+        first.leadingComments = [];
+      }
+    }
+
+    program.body.unshift(directive);
   }
 }

src/babel/transformation/transformers/other/strict.js

@@ -1,10 +1,10 @@
 import * as messages from "../../../messages";
 import * as t from "../../../types";
 
-export function Program(program) {
+export function Program(program, parent, scope, file) {
   var first = program.body[0];
   if (t.isExpressionStatement(first) && t.isLiteral(first.expression, { value: "use strict" })) {
-    program.body.shift();
+    file.set("existingStrictDirective", program.body.shift());
   }
 }
 

test/fixtures/transformation/es6-destructuring/array-unpack-optimisation/expected.js

@@ -1,6 +1,6 @@
+// opt
 "use strict";
 
-// opt
 var a = 1;
 var b = 2;
 var a = 1;

test/fixtures/transformation/es6-tail-call/ignore-reassigned/expected.js

@@ -1,8 +1,8 @@
-"use strict";
-
 // we need to deopt `test` if it's reassigned as we can't be certain of it's
 // state, ie. it could have been rebound or dereferenced
 
+"use strict";
+
 function test(exit) {
   if (exit) {
     return this.x;

test/fixtures/transformation/strict/leading-comments-with-existing/actual.js

@@ -0,0 +1,4 @@
+// comments
+"use strict";
+
+module.exports = {};

test/fixtures/transformation/strict/leading-comments-with-existing/expected.js

@@ -0,0 +1,4 @@
+// comments
+"use strict";
+
+module.exports = {};

test/fixtures/transformation/strict/leading-comments/actual.js

@@ -0,0 +1,3 @@
+// comments
+
+module.exports = {};

test/fixtures/transformation/strict/leading-comments/expected.js

@@ -0,0 +1,5 @@
+// comments
+
+"use strict";
+
+module.exports = {};