noUncheckedIndexedAccess gives unexpected behavior for string.split

[sla100]

TypeScript Version: 4.1.2

Search Terms:
4.1.2 split

Code

const [a] = 'a:b'.split(':');

Expected behavior:
typeof a should be string

string.split ALWAYS return string at first position

Actual behavior:
tyeof a is string|undefined

[ilogico]

Do you have noUncheckedIndexedAccess turned on?

[sla100]

Yes

[AlCalzone]

Then you did not read the release notes:

There's a reason this flag is NOT enabled by default.

[sla100]

But "more trickly" !== incorrect.
First item of string.split NEVER will be empty.

const [a,b] = someString.split(':');

should recognized:

a:string;
b:string|undefined;

[AlCalzone]

String.split returns an array. You're indexing into the array.

[RyanCavanaugh]

First item of string.split NEVER will be empty

This is actually tricky since it depends on the splitter argument. "".split("") is []

I don't think we want to bother special-casing the result of this function based on the split input.

[lo1tuma]

I don't think we want to bother special-casing the result of this function based on the split input.

I would be interested in the reasoning behind this. Why can’t we introduce a special case based on the separator argument as suggested in #42330?

[sharshuv-quotient]

This is actually tricky since it depends on the splitter argument. "".split("") is []

I don't think we want to bother special-casing the result of this function based on the split input.

So the only case that the first element in the array can be undefined is if both the string being split, and the separator are an empty string?

I think in almost all most situations this is not the case, and I wish typescript could handle that better:

Option 1: in situations in which one of the strings (the one being split, or the separator) are known in advanced to be not empty, typescript should interpret the first item of the resulting array to be defined.
Option 2: expose a dedicated tsconfig flag for this scenario, to be used together with noUncheckedIndexedAccess, in order to customize the behavior of split, so that the first item is always defined. I would use this flag as I can live with the edge case of ''.split('')

[RyanCavanaugh]

You can put this in your project

interface String {
    split(splitter: ""): string[];
    split(splitter: string): [string, ...string[]][];
}

We don't need a commandline flag for that.

[rpmccarter]

@RyanCavanaugh thanks for the tip! But just in case anyone else stumbles upon this, I believe the correct typing is

interface String {
    split(splitter: ""): string[];
    split(splitter: string): [string, ...string[]];
}

(no brackets at the end of the second split)