Defining the data type of one parameter based on another parameter within the same TypeScript function

Is there a way for a function to dynamically define the correct type of its second argument based on the value of the first parameter it receives?

Below is an example code snippet:

type MyType = {
    key1: string,
    key2: number,
    key3: Array<number>
}

function myFunction(x: keyof MyType, y: MyType[x]) {
    // function logic
    // ...
}

myFunction('key1', 'some string');  // This works
myFunction('key1', 1);  // This gives error
myFunction('key2', 1);  // This works
myFunction('key3', 'another string');  // This gives error

If this behavior can be achieved, how can it be done?

Answer №1

A technique to achieve this is by introducing two generic type parameters within the function (referred to as KeyType and ValueType) where KeyType is utilized in the definition of ValueType, as illustrated below:

function myFunction<
    KeyType extends keyof MyType,
    ValueType extends MyType[KeyType]
>(x: KeyType, y: ValueType) {
    // function logic
    // ...
}

It is not necessary to explicitly specify type arguments when calling the function; TypeScript can typically deduce them from usage, ensuring all examples operate as intended without requiring modifications.

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