Dynamic key access in object literal results in widened typescript signature

Question

For the following code:

const x = {
    a: 'c',
    b: 'd'
};

const y = {
    [x.a]: 'e',
}

the generated types are:

typeof x -> {
    a: string,
    b: string
}

typeof y -> {
  [x: string]: string
}

Expected type:

typeof y -> {
  c: string
}

Similar issue on SO has a solution which is not relevant here

Found the reported issue on Github which says this was fixed but is somehow not working

Harsh Vyas · Accepted Answer · 2019-10-18 05:31:01Z

4

That's because typeof x.a is actually a string. Here, x is constant but the value of x.a can be changed to any string value.

If the value of x.a is not going to change then a possible solution(using const assertion added in typescript version 3.4):

const x = {
    a: 'c',
    b: 'd'
} as const;

const y = {
    [x.a]: 'e',
}

typeof y -> {
    c: string
}

edited Oct 18, 2019 at 5:31

answered Oct 17, 2019 at 11:03

Harsh Vyas

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3 Comments

Urvashi Gupta Over a year ago

Thank you! But to note, this will work with version 3.4 and above. For older ones, we will have to do something like: const x = { a: 'c' as 'c', b: 'd' as 'd' }. Could you please update the answer to reflect the version information?

Harsh Vyas Over a year ago

Thanks, I will do it.

altocumulus Over a year ago

@HarshVyas Your edit introduced a typo: you mean v 3.4 not 4.3!

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