How to type an array of strings mapped to an object where keys and their values are the array value

Question

Here is what I'm trying to achieve

function Enum<T extends string>(arr: T[]): Record<T, T> {
  return arr.reduce((next, key, index) => {
    next[key] = key
    return next
  }, {} as Record<T, T>)
}
const Codes = Enum([
  'foo',
  'bar'
])

I want typescript to know that Codes.foo is only 'foo'. Right now it thinks it can be 'foo' | 'bar'

I'm aware of Enums. This is more of curiosity question.

cyr_x · Accepted Answer · 2020-03-23 09:30:58Z

1

You should use { [K in T]: K } instead of Record<T, T> to tell typescript that the key is actually the value:

function Enum<T extends string>(o: T[])  {
  return o.reduce((next, key) => {
    next[key] = key;
    return next;
  }, {} as {[K in T]: K});
}

answered Mar 23, 2020 at 9:30

cyr_x

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