Typescript: strictNullCheck Errors caused by accessing array attributes like `.pop` in a while loop

[TheLarkInn]

Edit by @DanielRosenwasser: This issue is about JavaScript files.

---

TypeScript Version: 2.9.0-dev.20180412

Search Terms: .pop, strictNullChecks while(), loop, array, possibly, undefined

Code

const queue = [
    () => { console.log("hello") }
];

while (queue.length) {
    queue.pop()();
}

Expected behavior:
That because the while loop is checking the length of an iterable is first > 0, that array methods are callable and valid with out errors.
Actual behavior:
Error message: Cannot invoke an object which is possibly 'undefined'.
Playground Link:
https://www.typescriptlang.org/play/#src=const%20queue%20%3D%20%5B%0D%0A%20%20%20%20()%20%3D%3E%20%7B%20console.log(%22hello%22)%20%7D%0D%0A%5D%3B%0D%0A%0D%0Awhile%20(queue.length)%20%7B%0D%0A%20%20%20%20queue.pop()()%3B%0D%0A%7D

Relevant Information
This is a pretty common practice in webpack to use a queue to perform breadth first search traversals of our dependency graph in the fastest means possible. So it would be nice to not have to @ts-ignore this line

[jack-williams]

Seems like a good use-case for a definite assignment assertion.

const queue = [
    () => { console.log("hello") }
];

while (queue.length) {
    queue.pop()!(); // <--- !
}

I think applying narrowing to this case might be tricky because of possible side effects.

[DanielRosenwasser]

Sean didn't mention it, but this was in a JavaScript file, and we don't have non-null assertion syntax there.

[DanielRosenwasser]

Just opened up #23405.

[jack-williams]

Ah sorry. Then my suggestion is redundant.

[mhegazy]

Seems like there are two ways to go here, either change the definition of pop to be "optimistic" and not return undefined, that will be inline with array indexing not containing undefined, or go with the more explicit option in #23405. The first approach seems rather targeted, and i am sure this will come up again. so #23405 seems like a better investment here.

The underlying issue is discussed in https://github.com/Microsoft/TypeScript/issues/9998.

Closing in favor of #23405.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.