Typescript key value pair to conditional object

Question

Assume I have a type

type MyType = {
   'KEY1': 'ValueForKey1';
   'KEY2': 'ValueForKey2';
   'KEY3': 'ValueForKey3';
}

and I have function which takes values from MyType as {key: 'KEY1', value: 'ValueForKey1'}

function myFunc(entry: MyParamType<MyType>) {

}

Where I defined MyParamType as

type MyParamType<T> = {
    key: keyof T;
    value: T[keyof T]
};

This works fine for

myFunc({
    key: 'KEY1',
    value: 'ValueForKey1'
})

and fails as expected for

myFunc({
    key: 'KEY1',
    value: 'ValueForKey5' // should fail
})

The issue I am facing is it can take the value of any key instead of that particular key So

myFunc({
    key: 'KEY1',
    value: 'ValueForKey2'
})

succeeds instead of failing.

How do I add the conditional restriction that only the corresponding key should succeed?

Playground link.

akuiper · Accepted Answer · 2021-07-05 00:47:01Z

2

You can define your method as:

function myFunc<K extends keyof MyType, V extends MyType[K]>(
  entry: {key: K, value: V}) {
// function body
}

Where you define two generic types K and V where V extends MyType[K], i.e. the value corresponds to the key.

See playground

answered Jul 5, 2021 at 0:47

akuiper

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CertainPerformance · Accepted Answer · 2021-07-05 00:43:37Z

2

Adding a second type argument - the key - will do what you want. Then you can use it instead of T[keyof T]:

type MyType = {
  'KEY1': 'ValueForKey1';
  'KEY2': 'ValueForKey2';
  'KEY3': 'ValueForKey3';
}
function myFunc<T extends Partial<MyType>, K extends keyof T>(entry: {
  key: K,
  value: T[K]
}) {

}

// Fails
myFunc({
  key: 'KEY1',
  value: 'ValueForKey2'
})

// Works
myFunc({
  key: 'KEY1',
  value: 'ValueForKey1'
})

answered Jul 5, 2021 at 0:43

CertainPerformance

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