What is the most effective way to determine the data type of a value associated with a key in an interface?

In my TypeScript interface, I have defined the following structure:

MyInterface {
  'key1': number | string;
  'key2': string;
  'key3': SomeOtherInterface;
}

I am looking to create a new type that utilizes the properties of MyInterface:

MyType<K = keyof MyInterface> = {
  'key': K;
  'value': MyInterface[K];
  'somethingElse': string
}

My goal is to ensure that if the key in MyType is 'key1', then the value must be of type number | string, and if the key is 'key2', the value must be of type string, and so on. However, the approach above results in an error: Type 'K' cannot be used to index type 'MyInterface'. How should I go about defining MyType?

typescript

Answer №1

In order to incorporate keys from the MyInterface into the MyType interface, it is necessary to utilize the extend keyword on MyType:

interface MyInterface {
  'key1': number | string;
  'key2': string;
  'key3': SomeOtherInterface;
}
interface MyType extends MyInterface {
  'key': K;
  'value': MyInterface[K];
  'somethingElse': string
}

const myVariabel: MyType;

As a result, the variable myVariabel now contains both the keys from MyInterface and the newly added keys. If there is a need to make a key from MyInterface optional, it can be achieved by extending the type with Partial<MyInterface>

Answer №2

const objWithNumbers: { [key: string]: number } = {
    apple: 1187,
    orange: 456
}

objWithNumbers['apple'] = 1187;
objWithNumbers.apple = 1187;

let fruitA = objWithNumbers['myFruit'];
let fruitB = objWithNumbers.myKey;

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