Conditional type of second function argument

Question

I have following function:

function doSomething(param1: string, param2: string) {
  return param1 + param2;
}

also I have json based type with structure looking similar to this:

a1: {
  b1: 'something1',
  b2: 'something2',
  b3: 'something3'
},
a2: {
  c1: 'something4',
  c2: 'something5'
}

...etc

I want nth argument of mentioned function to be literal of nth deep elements, so if first argument is 'a1', second should be 'b1' | 'b2' | 'b3', and if first argument is 'a2', second should be 'c1' | 'c2'.

For first argument I've made simple keyof typeof data type, which is working great:

// data is imported json
type FirstArg = keyof typeof data;

For second I was trying generic type like this, but without success:

type SecondArg<T extends FirstArg> = keyof typeof data[T];

Any chance to do that?

Titian Cernicova-Dragomir · Accepted Answer · 2019-07-01 09:37:09Z

3

You can't do it for any number of arguments, you can define overloads for up to a specific depth:

const data = {
    a1: {
        b1: 'something1',
        b2: 'something2',
        b3: 'something3'
    },
    a2: {
        c1: 'something4',
        c2: 'something5'
    }
}
type DataType = typeof data;
function doSomething<K extends keyof DataType, K2 extends keyof DataType[K]>(p1: K, p2: K2, p3: keyof DataType[K][K2]): string
function doSomething<K extends keyof DataType>(p1: K, p2: keyof DataType[K]): string
function doSomething(p1: keyof DataType): string
function doSomething(...keys: PropertyKey[]) {
    return keys.join('.');
}

doSomething("a1", "b1")
doSomething("a1", "c2") // err

answered Jul 1, 2019 at 9:37

Titian Cernicova-Dragomir

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1 Comment

ichi Over a year ago

Thanks, that's sufficient solution for my problem.

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