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strictNullCheck Errors caused by accessing array attributes like .pop in a while loop #23403
Description
Edit by Daniel Rosenwasser (@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
Activity
jack-williams commentedon Apr 13, 2018
Seems like a good use-case for a definite assignment assertion.I think applying narrowing to this case might be tricky because of possible side effects.DanielRosenwasser commentedon Apr 13, 2018
Sean didn't mention it, but this was in a JavaScript file, and we don't have non-null assertion syntax there.
DanielRosenwasser commentedon Apr 13, 2018
Just opened up #23405.
jack-williams commentedon Apr 13, 2018
Ah sorry. Then my suggestion is redundant.
mhegazy commentedon Apr 16, 2018
Seems like there are two ways to go here, either change the definition of
popto be "optimistic" and not returnundefined, that will be inline with array indexing not containingundefined, 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 #9998.
Closing in favor of #23405.
typescript-bot commentedon May 1, 2018
Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.