Typescript cannot assign object literal to a generic type

Question

I get a type error if I try to use an object literal with a generic type that has a type constraint, I am struggling to figure out why:

type WithKey = {
  readonly akey: string;
}

function listOfThings<T extends WithKey>(one: T) {
  // This is ok
  const result2: Array<T> = [one];

  //This errors with Type '{ akey: string; }' is not assignable to type 'T'.
  const result: Array<T> = [{ akey: 'foo' }]; 

  return result;
}

What do you expect the call const hmm = listOfThings({akey: "foo", bkey: 123}); to produce? — jcalz
– jcalz, Commented Dec 17, 2018 at 15:58
It has recently been proposed that the error, while accurate, should be made less confusing. — jcalz
– jcalz, Commented Dec 17, 2018 at 16:00

Aaron Beall · Accepted Answer · 2018-12-17 16:00:29Z

2

The reason it doesn't accept { akey: 'foo' } is because T only extends WithKey, so an object which is literally WithKey isn't necessarily assignable to T. For example:

listOfThings<{ akey: string; aflag: boolean }>()

{ akey: 'foo' } does not satisfy { akey: string; aflag: boolean }.

You could coerce the compiler using an assertion:

const result: Array<T> = [{ akey: 'foo' } as T];

But this is opening you up for a bug, as in the first example would compile but not be true at runtime. It seems like either this isn't what you want, or the types don't describe what you want.

answered Dec 17, 2018 at 16:00

Aaron Beall

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Derviş Kayımbaşıoğlu · Accepted Answer · 2018-12-17 15:19:00Z

1

{ akey: 'foo' } actually has type, it is not generic type. That's why it doesn't like your assignment.

compiler does not understand { akey: 'foo' } as WithKey type.

you can force typescript by casting for this assignment

const result: Array<T> = [<T>{ akey: 'foo' }];

answered Dec 17, 2018 at 15:19

Derviş Kayımbaşıoğlu

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1 Comment

jcalz Over a year ago

You can force it to do this but you shouldn't unless you know what you're doing. listOfThings({akey: "foo", bkey: 123}) will infer T as {akey: string, bkey: number}, and then {akey: string} is definitely not assignable to it.

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