fix: infer array type from outside of arrow function

[fedeci]

Q	A
Fixed Issues?	Fixes #12714
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

If ArrowFunctionExpression is a child of VariableDeclarator we can try to guess the execution status relative to another expression. This allows to infer array type from outside of an arrow function.

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[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/41225/

[nicolo-ribaudo]

Can you also add some tests at packages/babel-traverse/test/inference.js?

[merceyz]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/39050/

This REPL doesn't seem to work, I get this error:

Could not load Babel build #39050: Could not find valid @babel/standalone artifact in build #39050

[nicolo-ribaudo]

Yeah I know, it should be fixed by #12728 😅

[fedeci]

I don't know why the CI failed and I cannot reproduce it locally. If I run make -j tscheck flowcheck-ci lint-ci I just get a bunch of errors from babel-generator, but nothing from babel-traverse.

// A possible fix?
((target.parent as t.VariableDeclarator).id as t.Identifier).name

[merceyz]

Since #12728 has been merged could you rebase this PR so the REPL works please

[iabooo553]

EDIT by @nicolo-ribaudo: spam

[merceyz]

Tested and it does indeed fix the REPL provided in the issue, but when applied on the real code I extracted it from it doesn't as it returns the function that was created

• REPL

// Fixed in this PR
{
  const keys = [];

  const foo = () => {
    for (const key of keys) {
    }
  };
}

// Same output as before
{
  const foo = () => {
    const keys = [];

    const bar = () => {
      for (const key of keys) {
      }
    };

    return bar;
  };
}

[fedeci]

The return statement seems to break it🤔

[merceyz]

Can also be reproduced if the function is exported - REPL

const keys = [];

export const foo = () => {
  for (const key of keys) {
  }
};

[fedeci]

Exported function execution status is not guessed since #10417 and I think we should check it only when keys type is immutable.
However I just realised that this should not be transformed to a C-like for since keys is mutable and may no longer be an iterable when the function is executed. Right?

// Don't infer type
var keys = []
const foo = () => {
  for (const key of keys) {}
}

// Infer type
const keys = []
export const foo = () => {
  for (const key of keys) {}
}

// Infer type
export const bar = () => {
  const keys = []
  for (const key of keys) {}
}

// Don't infer type
var keys = []
export const baz = () => {
  for (const key of keys) {}
}

/cc @nicolo-ribaudo

[nicolo-ribaudo]

In that case keys is mutable, but it's not mutated so we can safely consider it as an array. It's similar to what we do in this example