Repository navigation
Typescript doesn't detect correct type in constant array. #13578
Description
TypeScript Version: 2.1.1 / nightly (2.2.0-dev.201xxxxx)
Code
let map = {};
[
['foo', false],
['bar', true],
['baz', true],
].forEach(entry => {
let id = map[entry[0]];
let booleanVar = entry[1];
});Expected behavior:
Typescript could detect that entry[0] is a string so a cast isn't needed.
Actual behavior:
Type 'true' cannot be used as an index type.
The problem is solved using a cast to string, or alternatively (in this case more elegantly) using a map.
Feel free to mark as wontfix or duplicate as appropriate.
Thanks!
Activity
RyanCavanaugh commentedon Jan 19, 2017
The problem is that we really have no idea that your array isn't mutated. Heterogeneous arrays are sometimes tuples and sometimes just arrays. In this case since you are using an array as a tuple (thus hopefully not calling
sort/push/popon it), you can add a type assertion to inform the checker:emilio commentedon Jan 19, 2017
Well, it's a temporary, so I expected Typescript to be able to auto-treat it as constant.
I guess you could whitelist
forEach(and some other methods) as aconstmethod or something, so you know it can't be mutated, but yeah, seems complex enough.Thanks for the reply! :)
RyanCavanaugh commentedon Jan 19, 2017
Not that it particularly matters (since we don't do it either way), but it's the possible mutation of
entrythat's the issue, which is definitely available to mutate. AndforEachon a temporary still isn't guaranteed to be immutable because the array itself is the third argument to theforEachcallback (JS: always out to get you).emilio commentedon Jan 19, 2017
Oh, of course, sorry :)