Type guards do not work on array access expressions

[OleksandrNechai]

TypeScript Version: 2.0.3

Code

let list = [1, '2'];
for (let i = 0; i < list.length; i++) {
    const j = i;
    if (typeof list[j] === 'string') {
        console.log(list[j].charAt(0)); // Property 'charAt' does not exist on type 'string | number'
    }
}

Expected behavior:
Successful compilation, since the type of list[j] is made clear with type guard

Actual behavior:
error TS2339: Property 'charAt' does not exist on type 'string | number'

[aluanhaddad]

I'm not sure if this is intended to work, it would stand to reason since j is const. But in the meantime the following workaround will get the job done

for (let i of list) {
    if (typeof i === 'string') {
        console.log(i.charAt(0));
    }
}

[mhegazy]

This is a duplicate of #10530

[RyanCavanaugh]

@mhegazy the sample doesn't compile under Nathan's PR

[mhegazy]

well.. it did in a previous iteration. the result was all array indexes were narrowed adding high perf and memory pressure. the current iteration of the PR, only narrows strings, this seems to be less expensive. we can discuss a better implementation.

[RyanCavanaugh]

Declined due to performance reasons. Since it should almost always be possible to write const j = list[i] instead, this shouldn't be too burdensome.