enable es6.spec.symbols by default

src/babel/transformation/transformers/deprecated.json

@@ -2,11 +2,11 @@
   "selfContained": "runtime",
   "unicode-regex": "regex.unicode",
   "spec.typeofSymbol": "es6.spec.symbols",
-  "es6.symbols": "es6.spec.symbols",
+  "es6.spec.symbols": "es6.symbols",
   "es6.blockScopingTDZ": "es6.spec.blockScoping",
 
   "utility.inlineExpressions": "minification.constantFolding",
   "utility.deadCodeElimination": "minification.deadCodeElimination",
   "utility.removeConsoleCalls": "minification.removeConsole",
-  "utility.removeDebugger": "minification.removeDebugger"
+  "utility.removeDebugger": "minification.removeDebugger",
 }

src/babel/transformation/transformers/es6/spec.symbols.js

@@ -1,37 +0,0 @@
-import * as t from "../../../types";
-
-export var metadata = {
-  optional: true
-};
-
-export function UnaryExpression(node, parent, scope, file) {
-  if (node._ignoreSpecSymbols) return;
-
-  if (node.operator === "typeof") {
-    var call = t.callExpression(file.addHelper("typeof"), [node.argument]);
-    if (this.get("argument").isIdentifier()) {
-      var undefLiteral = t.literal("undefined");
-      var unary = t.unaryExpression("typeof", node.argument);
-      unary._ignoreSpecSymbols = true;
-      return t.conditionalExpression(
-        t.binaryExpression("===", unary, undefLiteral),
-        undefLiteral,
-        call
-      );
-    } else {
-      return call;
-    }
-  }
-}
-
-export function BinaryExpression(node, parent, scope, file) {
-  if (node.operator === "instanceof") {
-    return t.callExpression(file.addHelper("instanceof"), [node.left, node.right]);
-  }
-}
-
-export function VariableDeclaration(node) {
-  if (node._generated) this.skip();
-}
-
-export { VariableDeclaration as FunctionDeclaration };

src/babel/transformation/transformers/es6/symbols.js

@@ -0,0 +1,44 @@
+import * as t from "../../../types";
+
+export var metadata = {
+  group: "builtin-pre"
+};
+
+const BINARY_COMPARISON_OPERATORS = ["!=", "==", "!==", "==="];
+
+export function UnaryExpression(node, parent, scope, file) {
+  if (node._ignoreSpecSymbols || node.operator !== "typeof") return;
+
+  if (t.isBinaryExpression(parent) && BINARY_COMPARISON_OPERATORS.indexOf(parent.operator) >= 0 && t.isLiteral(parent.right)) {
+    // don't touch binary comparisons with a string that doesn't equal symbol, we can statically determine
+    // that this condition will never be met which means faster code!
+    if (parent.right.value !== "symbol") return;
+  }
+
+
+  var call = t.callExpression(file.addHelper("typeof"), [node.argument]);
+  if (this.get("argument").isIdentifier()) {
+    var undefLiteral = t.literal("undefined");
+    var unary = t.unaryExpression("typeof", node.argument);
+    unary._ignoreSpecSymbols = true;
+    return t.conditionalExpression(
+      t.binaryExpression("===", unary, undefLiteral),
+      undefLiteral,
+      call
+    );
+  } else {
+    return call;
+  }
+}
+
+export function BinaryExpression(node, parent, scope, file) {
+  if (node.operator === "instanceof") {
+    return t.callExpression(file.addHelper("instanceof"), [node.left, node.right]);
+  }
+}
+
+export function VariableDeclaration(node) {
+  if (node._generated) this.skip();
+}
+
+export { VariableDeclaration as FunctionDeclaration };

src/babel/transformation/transformers/index.js

@@ -16,6 +16,7 @@ export default {
   "spec.functionName":                     require("./spec/function-name"),
   "es6.spec.templateLiterals":             require("./es6/spec.template-literals"),
   "es6.templateLiterals":                  require("./es6/template-literals"),
+  "es6.symbols":                           require("./es6/symbols"),
 
   //- builtin-basic
   // this is where the bulk of the ES6 transformations take place, none of them require traversal state
@@ -51,7 +52,6 @@ export default {
   "es7.exportExtensions":                  require("./es7/export-extensions"),
   "spec.protoToAssign":                    require("./spec/proto-to-assign"),
   "es7.doExpressions":                     require("./es7/do-expressions"),
-  "es6.spec.symbols":                      require("./es6/spec.symbols"),
   "es7.functionBind":                      require("./es7/function-bind"),
   "spec.undefinedToVoid":                  require("./spec/undefined-to-void"),