Why typescript complains object's key is undefined even if the previous command sets a value to that key?

Question

type MaybeThereIsAValue = {
  [p: string]: string | undefined
}

...

let bar: MaybeThereIsAValue = {};
const key = "carpe";
bar[key] = "diem";

const why = bar[key];
// why is string | undefined

Why is why string | undefined since I am literally assigning a value to bar[key] in the previous line ?

How can I avoid it ?

Check example here

Why I can't get the type string | undefined, my why is type of 'string' — zixiCat
– zixiCat, Commented Apr 3, 2021 at 10:27
@zixiCat Don't know. When I hover my mouse over the why, my IDE says it's string | undefined, check the attached image I just uploaded — but-why
– but-why, Commented Apr 3, 2021 at 10:29

zixiCat · Accepted Answer · 2021-04-05 02:04:38Z

4

I got why it would get that.

That is because strictNullChecks of tsconfig is set to be true, which default is false,

and const key = 'carpe' is only executed after the TS is compiled into JS, so TS doesn't know which key it is.

strictNullChecks: When type checking, take into account null and undefined.

So if you want to solve that, my two solutions are:

1. set the strictNullChecks of tsconfig to be false

2. Use ! non-null assertion operator

const why = bar[key]!;

Let's think about this situation:

let key = 'carpe';
bar[key] = 'diem';
// the next line would only be executed after the TS is compiled into JS.
key = 'test';  
// that is to say TS does not know the key is 'test' or 'carpe'
// so `why` is type of string or undefined

const why = bar[key];

If you set strictNullChecks to be false, why will always be type of string

edited Apr 5, 2021 at 2:04

answered Apr 3, 2021 at 10:48

zixiCat

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2 Comments

but-why Over a year ago

I get it to work with the ! at the end but could you elaborate more ? I mean why the distinction between null and undefined would cause such an issue since I explicitly declare it as string | undefined and not string | null | undefined ?

zixiCat Over a year ago

there no diff between them, strictNullChecks just let it consider the type of this object if contains null or undefined, if you set ____________________________________________________________________________________ type MaybeThereIsAValue = { [p: string]: string | undefined | null; }; ____________________________________________________________________________________ perhaps why is type of string | undefined | null

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