babel · nicolo-ribaudo · May 1, 2021 · May 1, 2021 · May 1, 2021 · May 1, 2021
diff --git a/packages/babel-generator/src/node/parentheses.ts b/packages/babel-generator/src/node/parentheses.ts
@@ -283,8 +283,13 @@ export function LogicalExpression(node: any, parent: any): boolean {
 
 export function Identifier(node: t.Identifier, parent: t.Node): boolean {
   // ECMAScript specifically forbids a for-of loop from starting with the
-  // token sequence "for ( async of", because it would be ambiguous with
-  // "for (async of => {};;)", so we need to add extra parentheses.
+  // token sequence `for (async of`, because it would be ambiguous with
+  // `for (async of => {};;)`, so we need to add extra parentheses.
+  //
+  // If the parent is a for-await-of loop (i.e. parent.await === true), the
+  // parentheses aren't strictly needed, but we add them anyway because
+  // some tools (including earlier Babel versions) can't parse
+  // `for await (async of [])` without them.
   return (
     node.name === "async" && t.isForOfStatement(parent) && node === parent.left
   );

diff --git a/packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js b/packages/babel-generator/test/fixtures/edgecase/for-async-of/input.js
@@ -4,4 +4,6 @@ for ((async) of async) async;
 
 for (\u0061sync of []);
 
+for (async.x of []);
+
 for (async in []);
diff --git a/packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js b/packages/babel-generator/test/fixtures/edgecase/for-async-of/output.js
@@ -4,4 +4,6 @@ for ((async) of async) async;
 
 for ((async) of []);
 
+for (async.x of []);
+
 for (async in []);
diff --git a/packages/babel-generator/test/fixtures/edgecase/for-await-async-of/input.js b/packages/babel-generator/test/fixtures/edgecase/for-await-async-of/input.js
@@ -0,0 +1,11 @@
+async function f() {
+  for await (async of []);
+
+  for await (async of async) async;
+
+  for await ((async) of []);
+
+  for await (async.x of []);
+
+  for await (\u0061sync of []);
+}
diff --git a/packages/babel-generator/test/fixtures/edgecase/for-await-async-of/output.js b/packages/babel-generator/test/fixtures/edgecase/for-await-async-of/output.js
@@ -0,0 +1,11 @@
+async function f() {
+  for await ((async) of []);
+
+  for await ((async) of async) async;
+
+  for await ((async) of []);
+
+  for await (async.x of []);
+
+  for await ((async) of []);
+}
diff --git a/packages/babel-parser/src/parser/error-message.js b/packages/babel-parser/src/parser/error-message.js
@@ -61,6 +61,7 @@ export const ErrorMessages = makeErrorTemplates(
       "'from' is not allowed as an identifier after 'export default'.",
     ForInOfLoopInitializer:
       "'%0' loop variable declaration may not have an initializer.",
+    ForOfAsync: "The left-hand side of a for-of loop may not be 'async'.",
     ForOfLet: "The left-hand side of a for-of loop may not start with 'let'.",
     GeneratorInSingleStatementContext:
       "Generators can only be declared at the top level or inside a block.",

diff --git a/packages/babel-parser/src/parser/expression.js b/packages/babel-parser/src/parser/expression.js
@@ -1032,7 +1032,16 @@ export default class ExpressionParser extends LValParser {
               true,
             );
           } else if (this.match(tt.name)) {
-            return this.parseAsyncArrowUnaryFunction(id);
+            // If the next token begins with "=", commit to parsing an async
+            // arrow function. (Peeking ahead for "=" lets us avoid a more
+            // expensive full-token lookahead on this common path.)
+            if (this.lookaheadCharCode() === charCodes.equalsTo) {
+              return this.parseAsyncArrowUnaryFunction(id);
+            } else {
+              // Otherwise, treat "async" as an identifier and let calling code
+              // deal with the current tt.name token.
+              return id;
+            }
           } else if (this.match(tt._do)) {
             return this.parseDo(true);
           }

diff --git a/packages/babel-parser/src/parser/statement.js b/packages/babel-parser/src/parser/statement.js
@@ -539,13 +539,33 @@ export default class StatementParser extends ExpressionParser {
       return this.parseFor(node, init);
     }
 
+    // Check whether the first token is possibly a contextual keyword, so that
+    // we can forbid `for (async of` if this turns out to be a for-of loop.
+    const startsWithUnescapedName =
+      this.match(tt.name) && !this.state.containsEsc;
+
     const refExpressionErrors = new ExpressionErrors();
     const init = this.parseExpression(true, refExpressionErrors);
     const isForOf = this.isContextual("of");
-    if (isForOf || this.match(tt._in)) {
-      if (isForOf && startsWithLet) {
+    if (isForOf) {
+      // Check for leading tokens that are forbidden in for-of loops:
+      if (startsWithLet) {
         this.raise(init.start, Errors.ForOfLet);
+      } else if (
+        // `for await (async of []);` is allowed.
+        awaitAt === -1 &&
+        startsWithUnescapedName &&
+        init.type === "Identifier" &&
+        init.name === "async"
+      ) {
+        // This catches the case where the `async` in `for (async of` was
+        // parsed as an identifier. If it was parsed as the start of an async
+        // arrow function (e.g. `for (async of => {} of []);`), the LVal check
+        // further down will raise a more appropriate error.
+        this.raise(init.start, Errors.ForOfAsync);
       }
+    }
+    if (isForOf || this.match(tt._in)) {
       this.toAssignable(init, /* isLHS */ true);
       const description = isForOf ? "for-of statement" : "for-in statement";
       this.checkLVal(init, description);

diff --git a/packages/babel-parser/test/fixtures/es2017/async-functions/export-invalid/options.json b/packages/babel-parser/test/fixtures/es2017/async-functions/export-invalid/options.json
@@ -1,4 +1,3 @@
 {
-  "sourceType": "module",
-  "throws": "Unexpected token, expected \"=>\" (1:31)"
-}
+  "sourceType": "module"
+}
diff --git a/packages/babel-parser/test/fixtures/es2017/async-functions/export-invalid/output.json b/packages/babel-parser/test/fixtures/es2017/async-functions/export-invalid/output.json
@@ -0,0 +1,46 @@
+{
+  "type": "File",
+  "start":0,"end":36,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":36}},
+  "errors": [
+    "SyntaxError: Missing semicolon. (1:20)",
+    "SyntaxError: Missing semicolon. (1:33)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":36,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":36}},
+    "sourceType": "module",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExportDefaultDeclaration",
+        "start":0,"end":20,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":20}},
+        "declaration": {
+          "type": "Identifier",
+          "start":15,"end":20,"loc":{"start":{"line":1,"column":15},"end":{"line":1,"column":20},"identifierName":"async"},
+          "name": "async"
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":21,"end":33,"loc":{"start":{"line":1,"column":21},"end":{"line":1,"column":33}},
+        "expression": {
+          "type": "CallExpression",
+          "start":21,"end":33,"loc":{"start":{"line":1,"column":21},"end":{"line":1,"column":33}},
+          "callee": {
+            "type": "Identifier",
+            "start":21,"end":30,"loc":{"start":{"line":1,"column":21},"end":{"line":1,"column":30},"identifierName":"functionX"},
+            "name": "functionX"
+          },
+          "arguments": []
+        }
+      },
+      {
+        "type": "BlockStatement",
+        "start":34,"end":36,"loc":{"start":{"line":1,"column":34},"end":{"line":1,"column":36}},
+        "body": [],
+        "directives": []
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/...-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow-semicolon/input.js b/...-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow-semicolon/input.js
@@ -0,0 +1,3 @@
+async function f() {
+  for await (async of => {};;);
+}
diff --git a/...ser/test/fixtures/es2018/async-generators/for-await-async-of-arrow-semicolon/options.json b/...ser/test/fixtures/es2018/async-generators/for-await-async-of-arrow-semicolon/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Unexpected token (2:6)"
+}
diff --git a/...ages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow/input.js b/...ages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow/input.js
@@ -0,0 +1,3 @@
+async function f() {
+  for await (async of => {} of x);
+}
diff --git a/...s/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow/output.json b/...s/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of-arrow/output.json
@@ -0,0 +1,69 @@
+{
+  "type": "File",
+  "start":0,"end":57,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+  "errors": [
+    "SyntaxError: Invalid left-hand side in for-of statement. (2:13)"
+  ],
+  "program": {
+    "type": "Program",
+    "start":0,"end":57,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "FunctionDeclaration",
+        "start":0,"end":57,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+        "id": {
+          "type": "Identifier",
+          "start":15,"end":16,"loc":{"start":{"line":1,"column":15},"end":{"line":1,"column":16},"identifierName":"f"},
+          "name": "f"
+        },
+        "generator": false,
+        "async": true,
+        "params": [],
+        "body": {
+          "type": "BlockStatement",
+          "start":19,"end":57,"loc":{"start":{"line":1,"column":19},"end":{"line":3,"column":1}},
+          "body": [
+            {
+              "type": "ForOfStatement",
+              "start":23,"end":55,"loc":{"start":{"line":2,"column":2},"end":{"line":2,"column":34}},
+              "await": true,
+              "left": {
+                "type": "ArrowFunctionExpression",
+                "start":34,"end":48,"loc":{"start":{"line":2,"column":13},"end":{"line":2,"column":27}},
+                "id": null,
+                "generator": false,
+                "async": true,
+                "params": [
+                  {
+                    "type": "Identifier",
+                    "start":40,"end":42,"loc":{"start":{"line":2,"column":19},"end":{"line":2,"column":21},"identifierName":"of"},
+                    "name": "of"
+                  }
+                ],
+                "body": {
+                  "type": "BlockStatement",
+                  "start":46,"end":48,"loc":{"start":{"line":2,"column":25},"end":{"line":2,"column":27}},
+                  "body": [],
+                  "directives": []
+                }
+              },
+              "right": {
+                "type": "Identifier",
+                "start":52,"end":53,"loc":{"start":{"line":2,"column":31},"end":{"line":2,"column":32},"identifierName":"x"},
+                "name": "x"
+              },
+              "body": {
+                "type": "EmptyStatement",
+                "start":54,"end":55,"loc":{"start":{"line":2,"column":33},"end":{"line":2,"column":34}}
+              }
+            }
+          ],
+          "directives": []
+        }
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/packages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of/input.js b/packages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of/input.js
@@ -0,0 +1,3 @@
+async function f() {
+  for await (async of x);
+}
diff --git a/packages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of/output.json b/packages/babel-parser/test/fixtures/es2018/async-generators/for-await-async-of/output.json
@@ -0,0 +1,51 @@
+{
+  "type": "File",
+  "start":0,"end":48,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":48,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "FunctionDeclaration",
+        "start":0,"end":48,"loc":{"start":{"line":1,"column":0},"end":{"line":3,"column":1}},
+        "id": {
+          "type": "Identifier",
+          "start":15,"end":16,"loc":{"start":{"line":1,"column":15},"end":{"line":1,"column":16},"identifierName":"f"},
+          "name": "f"
+        },
+        "generator": false,
+        "async": true,
+        "params": [],
+        "body": {
+          "type": "BlockStatement",
+          "start":19,"end":48,"loc":{"start":{"line":1,"column":19},"end":{"line":3,"column":1}},
+          "body": [
+            {
+              "type": "ForOfStatement",
+              "start":23,"end":46,"loc":{"start":{"line":2,"column":2},"end":{"line":2,"column":25}},
+              "await": true,
+              "left": {
+                "type": "Identifier",
+                "start":34,"end":39,"loc":{"start":{"line":2,"column":13},"end":{"line":2,"column":18},"identifierName":"async"},
+                "name": "async"
+              },
+              "right": {
+                "type": "Identifier",
+                "start":43,"end":44,"loc":{"start":{"line":2,"column":22},"end":{"line":2,"column":23},"identifierName":"x"},
+                "name": "x"
+              },
+              "body": {
+                "type": "EmptyStatement",
+                "start":45,"end":46,"loc":{"start":{"line":2,"column":24},"end":{"line":2,"column":25}}
+              }
+            }
+          ],
+          "directives": []
+        }
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/packages/babel-parser/test/fixtures/es2021/for-async-of/dot/input.js b/packages/babel-parser/test/fixtures/es2021/for-async-of/dot/input.js
@@ -0,0 +1 @@
+for (async.x of y);
diff --git a/packages/babel-parser/test/fixtures/es2021/for-async-of/dot/output.json b/packages/babel-parser/test/fixtures/es2021/for-async-of/dot/output.json
@@ -0,0 +1,42 @@
+{
+  "type": "File",
+  "start":0,"end":19,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":19}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":19,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":19}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ForOfStatement",
+        "start":0,"end":19,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":19}},
+        "await": false,
+        "left": {
+          "type": "MemberExpression",
+          "start":5,"end":12,"loc":{"start":{"line":1,"column":5},"end":{"line":1,"column":12}},
+          "object": {
+            "type": "Identifier",
+            "start":5,"end":10,"loc":{"start":{"line":1,"column":5},"end":{"line":1,"column":10},"identifierName":"async"},
+            "name": "async"
+          },
+          "computed": false,
+          "property": {
+            "type": "Identifier",
+            "start":11,"end":12,"loc":{"start":{"line":1,"column":11},"end":{"line":1,"column":12},"identifierName":"x"},
+            "name": "x"
+          }
+        },
+        "right": {
+          "type": "Identifier",
+          "start":16,"end":17,"loc":{"start":{"line":1,"column":16},"end":{"line":1,"column":17},"identifierName":"y"},
+          "name": "y"
+        },
+        "body": {
+          "type": "EmptyStatement",
+          "start":18,"end":19,"loc":{"start":{"line":1,"column":18},"end":{"line":1,"column":19}}
+        }
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/packages/babel-parser/test/fixtures/es2021/for-async-of/escaped/input.js b/packages/babel-parser/test/fixtures/es2021/for-async-of/escaped/input.js
@@ -0,0 +1 @@
+for (\u0061sync of x);
diff --git a/packages/babel-parser/test/fixtures/es2021/for-async-of/escaped/output.json b/packages/babel-parser/test/fixtures/es2021/for-async-of/escaped/output.json
@@ -0,0 +1,32 @@
+{
+  "type": "File",
+  "start":0,"end":22,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":22}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":22,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":22}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ForOfStatement",
+        "start":0,"end":22,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":22}},
+        "await": false,
+        "left": {
+          "type": "Identifier",
+          "start":5,"end":15,"loc":{"start":{"line":1,"column":5},"end":{"line":1,"column":15},"identifierName":"async"},
+          "name": "async"
+        },
+        "right": {
+          "type": "Identifier",
+          "start":19,"end":20,"loc":{"start":{"line":1,"column":19},"end":{"line":1,"column":20},"identifierName":"x"},
+          "name": "x"
+        },
+        "body": {
+          "type": "EmptyStatement",
+          "start":21,"end":22,"loc":{"start":{"line":1,"column":21},"end":{"line":1,"column":22}}
+        }
+      }
+    ],
+    "directives": []
+  }
+}
diff --git a/packages/babel-parser/test/fixtures/es2021/for-async-of/for-statement/input.js b/packages/babel-parser/test/fixtures/es2021/for-async-of/for-statement/input.js
@@ -0,0 +1 @@
+for (async of => {};;);