Index Types

Using raw strings to convey type information:

function myFn<T, K extends keyof T>(
  collection: T[],
  propName: K,
) {
  // `propName` can only be a valid property i.e. `K` exists on `T`
}

E.g. the following function indexes a collection only the properties that are valid:

function indexBy<T, K extends keyof T>(
  collection: T[],
  propName: K,
) {
  return collection.reduce((dictionary, i) => {
    const key = i[propName];
    dictionary[key] = i;
    return dictionary;
  }, {} as any);
}

Also with return type:

function indexBy<T, K extends keyof T, G extends string | number>(
  collection: T[],
  propName: K,
): Record<G, T> {
  return collection.reduce((dictionary, i) => {
    const key = i[propName];
    dictionary[key] = i;
    return dictionary;
  }, {} as any);
}

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• Tag: typescript