`ArrayLiteral` instead of `ArrayBindingPattern` as loop variable

[knutwannheden]

🔎 Search Terms

"for loop", "array literal", "array binding pattern"

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about for loops
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-dev.20250612#code/DYUwLgBAZglgTgZzAGggkBjA9gOwCYDcAUFFnBABQDasiKamueAuhFlBFVQIzIBMzZFQDMyACzNmASggBvIhEURsOBFlAA6YFgDmFWklToVeKcQC+QA

💻 Code

let first, second;
for ([first, second] of [[1,2],[3,4]]) {
    console.log(first, second);
}

🙁 Actual behavior

The [first, second] expression in the example's for ... of loop is mapped to a ts.ArrayLiteral which gets assigned to the ts.ForOfStatement's initalizer property.

🙂 Expected behavior

Even though the variables are declared before the loop, I would have expected to see a ts.ArrayBindingPattern node in the AST.

Additional information about the issue

See also: https://ts-ast-viewer.com/#code/DYUwLgBAZglgTgZzAGggkBjA9gOwCYDcAUFFnBABQDasiKamueAuhFlBFVQIzIBMzZFQDMyACzNmASggBvIhEURsOBFlAA6YFgDmFWklToVeKcQC+QA

The same is true for object binding patterns, which get mapped to ts.ObjectLiteralExpressions rather than a ts.ObjectBindingPattern.

[RyanCavanaugh]

The meaning of a particular node kind is contextual.

[knutwannheden]

Thanks for the reply! Can you elaborate a bit on this or is there some document I can read which explains what you mean by this?

[RyanCavanaugh]

I'm not sure what kind of information you're looking for. The kind of a node alone doesn't always tell you enough information about exactly what it is -- the position of it in the tree is also a component that needs consideration.

In an example like

[x, y] = [a, b];

the LHS is also parsed as an ArrayLiteral, since otherwise the parser would have to backtrack once it encountered the =, which isn't efficient. A for/of can be thought of in the same way; it's a repeated destructuring assignment. So you could think of it as an ArrayLiteral or you could think of it as an ArrayBindingPattern; both are correct. It'd only be a bug if the code that interpreted the tree failed to take this into account, but it doesn't.

[knutwannheden]

Thanks for clarifying! I suspected that this is what you meant and I can see why it was done this way. I just wanted to make sure, as your reply was very terse.