Typing nested array of objects

Question

How can I define foo so that i don't have to specify as Constraint for each of the array items ?

type Constraint = {
    constraint: 'required' | 'equal'
}


function bar<K extends string>(d: Record<K, Constraint[]>) {
    return Object.keys(d).reduce((a, b) => (a[b] = '', a),
                                 {} as { [key: string]: string }) as Record<K, string>
}

const foo = { apple: [ { constraint: 'required' } ] }
// Error  Type 'string' is not assignable to type '"required" | "equal"'.
const baz = bar(foo)


// Works fine
const foo2 = { apple: [  { constraint: 'required' } as Constraint ] }
const baz2 = bar(foo2)

leonardfactory · Accepted Answer · 2020-04-27 12:00:52Z

3

When you type a string, TS recognizes it as a plain string, not like the 'required' literal. You can:

Use the object literal directly in the function call:

const baz = bar({ apple: [ { constraint: 'required' } ] })

This way inference is preserved

Declare the string as const, i.e.:

const foo = { apple: [ { constraint: 'required' as const } ] }

This lets know TS that you don't mean a generic string but the exact literal you typed. In older TS version as const is not present buy you could still use 'required' as 'required'

answered Apr 27, 2020 at 12:00

leonardfactory

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