Spec compatibility for template literals.

packages/babel-plugin-transform-es2015-template-literals/README.md

@@ -75,7 +75,7 @@ In loose mode, tagged template literal objects aren't frozen.
 
 `boolean`, defaults to `false`.
 
-This option wraps all template literal expressions with `String`. See [babel/babel#1065](https://github.com/babel/babel/issues/1065) for more info.
+This option combines all template literal expressions and quasis with `String.prototype.concat`.
 
 **In**
 
@@ -86,5 +86,5 @@ This option wraps all template literal expressions with `String`. See [babel/bab
 **Out**
 
 ```javascript
-"foo" + String(bar);
+"foo".concat(bar);
 ```

packages/babel-plugin-transform-es2015-template-literals/src/index.js

@@ -1,7 +1,28 @@
 export default function ({ types: t }) {
+
+  function buildConcatCallExressions(items) {
+    return items.reduce(function(left, right) {
+      const isString = t.isStringLiteral(right);
+      const canBeInserted = !left._withIdentifier || isString;
+
+      if (t.isCallExpression(left) && canBeInserted) {
+        left._withIdentifier = left._withIdentifier || !isString;
+        left.arguments.push(right);
+        return left;
+      }
+      const nextCall = t.callExpression(
+        t.memberExpression(left, t.identifier("concat")),
+        [right]
+      );
+      nextCall._withIdentifier = !isString;
+      return nextCall;
+    });
+  }
+
   return {
     visitor: {
       TaggedTemplateExpression(path, state) {
+
         const { node } = path;
         const { quasi } = node;
 
@@ -45,23 +66,28 @@ export default function ({ types: t }) {
           if (index < expressions.length) {
             const expr = expressions[index++];
             const node = expr.node;
-            if (state.opts.spec && !expr.isBaseType("string") && !expr.isBaseType("number")) {
-              nodes.push(t.callExpression(t.identifier("String"), [node]));
-            } else if (!t.isStringLiteral(node, { value: "" })) {
+            if (!t.isStringLiteral(node, { value: "" })) {
               nodes.push(node);
             }
           }
         }
 
         // since `+` is left-to-right associative
         // ensure the first node is a string if first/second isn't
-        if (!t.isStringLiteral(nodes[0]) && !t.isStringLiteral(nodes[1])) {
+        const considerSecondNode = state.opts.spec || !t.isStringLiteral(nodes[1]);
+        if (!t.isStringLiteral(nodes[0]) && considerSecondNode) {
           nodes.unshift(t.stringLiteral(""));
         }
-
         let root = nodes[0];
-        for (let i = 1; i < nodes.length; i++) {
-          root = t.binaryExpression("+", root, nodes[i]);
+
+        if (state.opts.spec) {
+          if (nodes.length > 1) {
+            root = buildConcatCallExressions(nodes);
+          }
+        } else {
+          for (let i = 1; i < nodes.length; i++) {
+            root = t.binaryExpression("+", root, nodes[i]);
+          }
         }
 
         path.replaceWith(root);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/escape-quotes/expected.js

@@ -1 +1 @@
-var t = "'" + String(foo) + "' \"" + String(bar) + "\"";
+var t = "'".concat(foo, "' \"").concat(bar, "\"");

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/functions/expected.js

@@ -1 +1 @@
-var foo = "test " + String(_.test(foo)) + " " + String(bar);
+var foo = "test ".concat(_.test(foo), " ").concat(bar);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/multiple/expected.js

@@ -1 +1 @@
-var foo = "test " + String(foo) + " " + String(bar);
+var foo = "test ".concat(foo, " ").concat(bar);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/only/expected.js

@@ -1 +1 @@
-var foo = "" + String(test);
+var foo = "".concat(test);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/single/expected.js

@@ -1 +1 @@
-var foo = "test " + String(foo);
+var foo = "test ".concat(foo);

packages/babel-plugin-transform-es2015-template-literals/test/fixtures/spec/statement/expected.js

@@ -1 +1 @@
-var foo = "test " + String(foo + bar);
+var foo = "test ".concat(foo + bar);

packages/babel-preset-es2015/test/fixtures/preset-options/spec/expected.js

@@ -1,7 +1,7 @@
 "use strict";
 
 var _this = undefined;
-"1" + String(a);
+"1".concat(a);
 
 (function () {
   babelHelpers.newArrowCheck(this, _this);