implement optional TDZ - fixes #563

lib/6to5/transformation/file.js

@@ -54,7 +54,9 @@ File.helpers = [
   "get",
   "set",
   "class-call-check",
-  "object-destructuring-empty"
+  "object-destructuring-empty",
+  "temporal-undefined",
+  "temporal-assert-defined"
 ];
 
 File.validOptions = [

lib/6to5/transformation/templates/default-parameter.js

@@ -1 +1 @@
-var VARIABLE_NAME = ARGUMENTS[ARGUMENT_KEY] === undefined ? DEFAULT_VALUE : ARGUMENTS[ARGUMENT_KEY];
+let VARIABLE_NAME = ARGUMENTS[ARGUMENT_KEY] === undefined ? DEFAULT_VALUE : ARGUMENTS[ARGUMENT_KEY];

lib/6to5/transformation/templates/temporal-assert-defined.js

@@ -0,0 +1,6 @@
+(function (val, name, undef) {
+  if (val === undef) {
+    throw new ReferenceError(name + " is not defined - temporal dead zone");
+  }
+  return true;
+})

lib/6to5/transformation/templates/temporal-undefined.js

@@ -0,0 +1 @@
+({})

lib/6to5/transformation/transformers/es6/block-scoping-tdz.js

@@ -12,22 +12,18 @@ var visitor = {
     // declared node is different in this scope
     if (scope.getBinding(node.name) !== declared) return;
 
-    var declaredLoc  = declared.loc;
-    var referenceLoc = node.loc;
+    var assert = t.callExpression(
+      state.file.addHelper("temporal-assert-defined"),
+      [node, t.literal(node.name), state.file.addHelper("temporal-undefined")]
+    );
 
-    if (!declaredLoc || !referenceLoc) return;
+    this.skip();
 
-    // does this reference appear on a line before the declaration?
-    var before = referenceLoc.start.line < declaredLoc.start.line;
-
-    if (referenceLoc.start.line === declaredLoc.start.line) {
-      // this reference appears on the same line
-      // check it appears before the declaration
-      before = referenceLoc.start.col < declaredLoc.start.col;
-    }
-
-    if (before) {
-      throw state.file.errorWithNode(node, "Temporal dead zone - accessing a variable before it's initialized");
+    if (t.isAssignmentExpression(parent) || t.isUpdateExpression(parent)) {
+      if (parent._ignoreBlockScopingTDZ) return;
+      this.parentPath.replaceNode(t.sequenceExpression([assert, parent]));
+    } else {
+      return t.logicalExpression("&&", assert, node);
     }
   }
 };

lib/6to5/transformation/transformers/es6/block-scoping.js

@@ -17,7 +17,7 @@ var isLet = function (node, parent) {
   if (node.kind !== "let") return false;
 
   // https://github.com/6to5/6to5/issues/255
-  if (!t.isFor(parent) || t.isFor(parent) && parent.left !== node) {
+  if (isLetInitable(node, parent)) {
     for (var i = 0; i < node.declarations.length; i++) {
       var declar = node.declarations[i];
       declar.init = declar.init || t.identifier("undefined");
@@ -29,6 +29,10 @@ var isLet = function (node, parent) {
   return true;
 };
 
+var isLetInitable = function (node, parent) {
+  return !t.isFor(parent) || t.isFor(parent) && parent.left !== node;
+};
+
 var isVar = function (node, parent) {
   return t.isVariableDeclaration(node, { kind: "var" }) && !isLet(node, parent);
 };
@@ -39,8 +43,26 @@ var standardizeLets = function (declars) {
   }
 };
 
-exports.VariableDeclaration = function (node, parent) {
-  isLet(node, parent);
+exports.VariableDeclaration = function (node, parent, scope, file) {
+  if (!isLet(node, parent)) return;
+
+  if (isLetInitable(node) && file.transformers["es6.blockScopingTDZ"].canRun()) {
+    var nodes = [node];
+
+    for (var i = 0; i < node.declarations.length; i++) {
+      var decl = node.declarations[i];
+      if (decl.init) {
+        var assign = t.assignmentExpression("=", decl.id, decl.init);
+        assign._ignoreBlockScopingTDZ = true;
+        nodes.push(t.expressionStatement(assign));
+      }
+      decl.init = file.addHelper("temporal-undefined");
+    }
+
+    node._blockHoist = 2;
+
+    return nodes;
+  }
 };
 
 exports.Loop = function (node, parent, scope, file) {

lib/6to5/transformation/transformers/es6/parameters.default.js

@@ -23,7 +23,7 @@ var iifeVisitor = {
   }
 };
 
-exports.Function = function (node, parent, scope) {
+exports.Function = function (node, parent, scope, file) {
   if (!hasDefaults(node)) return;
 
   t.ensureBlock(node);
@@ -37,11 +37,26 @@ exports.Function = function (node, parent, scope) {
 
   var state = { iife: false, scope: scope };
 
+  var pushDefNode = function (left, right, i) {
+    var defNode = util.template("default-parameter", {
+      VARIABLE_NAME: left,
+      DEFAULT_VALUE: right,
+      ARGUMENT_KEY:  t.literal(i),
+      ARGUMENTS:     argsIdentifier
+    }, true);
+    file.checkNode(defNode);
+    defNode._blockHoist = node.params.length - i;
+    body.push(defNode);
+  };
+
   for (var i = 0; i < node.params.length; i++) {
     var param = node.params[i];
 
     if (!t.isAssignmentPattern(param)) {
-      lastNonDefaultParam = +i + 1;
+      lastNonDefaultParam = i + 1;
+      if (file.transformers["es6.blockScopingTDZ"].canRun()) {
+        pushDefNode(param, t.identifier("undefined"), i);
+      }
       continue;
     }
 
@@ -58,14 +73,7 @@ exports.Function = function (node, parent, scope) {
       }
     }
 
-    var defNode = util.template("default-parameter", {
-      VARIABLE_NAME: left,
-      DEFAULT_VALUE: right,
-      ARGUMENT_KEY:  t.literal(+i),
-      ARGUMENTS:     argsIdentifier
-    }, true);
-    defNode._blockHoist = node.params.length - i;
-    body.push(defNode);
+    pushDefNode(left, right, i);
   }
 
   // we need to cut off all trailing default parameters

lib/6to5/transformation/transformers/index.js

@@ -46,6 +46,9 @@ module.exports = {
 
   "es6.constants":                         require("./es6/constants"),
 
+  // needs to be before `es6.blockScoping` as default parameters have a TDZ
+  "es6.parameters.default":                require("./es6/parameters.default"),
+
   // needs to be before `_aliasFunction` due to block scopes sometimes being wrapped in a
   // closure
   "es6.blockScoping":                      require("./es6/block-scoping"),
@@ -53,7 +56,6 @@ module.exports = {
   // needs to be after `es6.blockScoping` due to needing `letReferences` set on blocks
   "es6.blockScopingTDZ":                   require("./es6/block-scoping-tdz"),
 
-  "es6.parameters.default":                require("./es6/parameters.default"),
   "es6.parameters.rest":                   require("./es6/parameters.rest"),
 
   "es6.destructuring":                     require("./es6/destructuring"),