Typescript error 2345 when passing array as variable instead of directly

Question

Question was already asked here but both the solutions provided don't address nor solve the issue

I was following some TypeScripts examples about generics, in the specific this one.

The example is pretty straightforward:

function pickObjectKeys<T, K extends keyof T>(obj: T, keys: K[]) {
  let result = {} as Pick<T, K>
  for (const key of keys) {
    if (key in obj) {
      result[key] = obj[key]
    }
  }
  return result
}

const language = {
  name: "TypeScript",
  age: 8,
  extensions: ['ts', 'tsx']
}

const ageAndExtensions = pickObjectKeys(language, ['age', 'extensions'])

This works. However, if passing the array as a variable, like so:

const keys = ['age', 'extensions']

...

pickObjectKeys(language, keys)

the TypeScript compiler gives error 2345:

Argument of type 'string[]' is not assignable to parameter of type '("age" | "extensions" | "name")[]'.
Type 'string' is not assignable to type '"age" | "extensions" | "name"'.

What's the catch? How to address this issue?

keys is inferred as string[], not ["age", "extensions"] or ("age" | "extensions")[]. — k.tten
– k.tten, Commented Oct 22, 2022 at 13:31
@caTS Ah, makes sense. So how would you address it? Assume that you may want to be able to pass a variable to the function — wtfzambo
– wtfzambo, Commented Oct 22, 2022 at 13:35
const keys : ("age" | "extensions")[] = ["age", "extensions"] or const keys = ["age" | "extensions"] as ("age" | "extensions")[]. Because if you just do const keys = ["age", "extensions"] it's totally legal to do keys.push("foobar") but that would cause an error in pickObjectKeys — derpirscher
– derpirscher, Commented Oct 22, 2022 at 13:36

k.tten · Accepted Answer · 2022-10-22 13:37:47Z

1

First, consider making keys readonly:

function pickObjectKeys<T, K extends keyof T>(obj: T, keys: readonly K[]) {

this will allow you to pass readonly arrays now (previously it would've errored).

The easiest way to get around this is to use a const assertion:

const keys = ['age', 'extensions'] as const; // readonly ["age", "extensions"]

pickObjectKeys(language, keys); // OK

but you could also cast to ["age", "extensions"] or ("age" | "extensions")[]. However, that does mean you would repeat yourself, so I suggest staying with as const.

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answered Oct 22, 2022 at 13:37

k.tten

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