How to get mapped type of nested property in array item?

Question

const arr = [
  { key: 'one', value: '1' },
  { key: 'two', value: '2' },
  { key: 'three', value: '3' },
] as const;

type Keys = GetKeys<typeof arr>;

How to define GetKeys so that Keys would be be 'one' | 'two' | 'three'?

Can you use built-in enum?

PM 77-1
– PM 77-1

2019-07-03 16:22:13 +00:00
Commented Jul 3, 2019 at 16:22 — PM 77-1
– PM 77-1, Commented Jul 3, 2019 at 16:22

jcalz · Accepted Answer · 2019-07-03 16:34:56Z

1

You just want to use a lookup type (also called "indexed access type") a few times to get the numeric-keyed elements from the array and then the "key"-keyed properties of those:

type GetKeys<T extends readonly {key: any}[]> = T[number]['key'];

type Keys = GetKeys<typeof arr>;
// type Keys = "one" | "two" | "three"

Hope that helps; good luck!

Link to code

answered Jul 3, 2019 at 16:34

jcalz

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