Interface type check with Typescript

Question

This question is the direct analogon to Class type check with TypeScript

I need to find out at runtime if a variable of type any implements an interface. Here\'s my

Answer 1

Based on Fenton's answer, here's my implementation of a function to verify if a given object has the keys an interface has, both fully or partially.

Depending on your use case, you may also need to check the types of each of the interface's properties. The code below doesn't do that.

function implementsTKeys(obj: any, keys: (keyof T)[]): obj is T {
    if (!obj || !Array.isArray(keys)) {
        return false;
    }

    const implementKeys = keys.reduce((impl, key) => impl && key in obj, true);

    return implementKeys;
}

Example of usage:

interface A {
    propOfA: string;
    methodOfA: Function;
}

let objectA: any = { propOfA: '' };

// Check if objectA partially implements A
let implementsA = implementsTKeys(objectA, ['propOfA']);

console.log(implementsA); // true

objectA.methodOfA = () => true;

// Check if objectA fully implements A
implementsA = implementsTKeys(objectA, ['propOfA', 'methodOfA']);

console.log(implementsA); // true

objectA = {};

// Check again if objectA fully implements A
implementsA = implementsTKeys(objectA, ['propOfA', 'methodOfA']);

console.log(implementsA); // false, as objectA now is an empty object