Valid JavaScript with in inside for produces syntax errors and invalid JavaScript output

[evanw]

Bug Report

🔎 Search Terms

for in syntax error

🕗 Version & Regression Information

• This is the behavior in every version available in the playground

⏯ Playground Link

Playground link with relevant code

💻 Code

for (let [x = 'a' in {}] of [[]]) console.log(x)
for (let {x = 'a' in {}} of [{}]) console.log(x)

for (let [x = 'a' in {}] in { '': 0 }) console.log(x)
for (let {x = 'a' in {}} in { '': 0 }) console.log(x)

for (let [x = 'a' in {}] = []; !x; x = !x) console.log(x)
for (let {x = 'a' in {}} = {}; !x; x = !x) console.log(x)

🙁 Actual behavior

When run, this code should print false six times. Each false is the value of x which has been default-initialized.

🙂 Expected behavior

TypeScript incorrectly generates syntax errors and generates the following invalid JavaScript output:

"use strict";
for (let [x = 'a'] in {})
    ;
of[[]];
console.log(x);
for (let { x = 'a', in: {} } of [{}])
    console.log(x);
for (let [x = 'a'] in {})
    ;
 in { '': 0 };
console.log(x);
for (let { x = 'a', in: {} } in { '': 0 })
    console.log(x);
for (let [x = 'a'] in {})
    ;
[];
!x;
x = !x;
console.log(x);
for (let { x = 'a', in: {} } = {}; !x; x = !x)
    console.log(x);

This came up because I was using VSCode to look at some JavaScript language tests and it was incorrectly showing syntax errors. I'm reporting this because it doesn't seem ideal that VSCode's JavaScript language support would treat valid JavaScript as invalid, and also because I believe the fix for this is relatively straightforward (probably something to do with DisallowInContext in the parser?).

[MartinJohns]

Sounds like a duplicate of #9978. Used search terms: in for in:title label:"Won't Fix"

Similar issues occasionally pop up and get closed as Won't Fix. There's really no point in spending time to make something work that's realistically never used.

[evanw]

I believe this issue is different because a) TypeScript is incorrectly generating syntax errors in valid JavaScript files in this case but not in #9978 and b) TypeScript generates garbled JavaScript output in this case but not in #9978. I think it's also worth fixing because the fix is pretty trivial:

diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index f49b48bb44..c70bf754d1 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -7411,7 +7411,7 @@ namespace Parser {
     function parseObjectBindingPattern(): ObjectBindingPattern {
         const pos = getNodePos();
         parseExpected(SyntaxKind.OpenBraceToken);
-        const elements = parseDelimitedList(ParsingContext.ObjectBindingElements, parseObjectBindingElement);
+        const elements = allowInAnd(() => parseDelimitedList(ParsingContext.ObjectBindingElements, parseObjectBindingElement));
         parseExpected(SyntaxKind.CloseBraceToken);
         return finishNode(factory.createObjectBindingPattern(elements), pos);
     }
@@ -7419,7 +7419,7 @@ namespace Parser {
     function parseArrayBindingPattern(): ArrayBindingPattern {
         const pos = getNodePos();
         parseExpected(SyntaxKind.OpenBracketToken);
-        const elements = parseDelimitedList(ParsingContext.ArrayBindingElements, parseArrayBindingElement);
+        const elements = allowInAnd(() => parseDelimitedList(ParsingContext.ArrayBindingElements, parseArrayBindingElement));
         parseExpected(SyntaxKind.CloseBracketToken);
         return finishNode(factory.createArrayBindingPattern(elements), pos);
     }