Function fnApply

The fnApply function calls the specified fn function with the given thisArg as the this value, and the optional argArray arguments provided as an array (or an Array-Like Object).

Normally, when calling a function, the value of this inside the function is the object that the function was accessed on. With fnApply(), you can assign an arbitrary value as this when calling an existing function, without first attaching the function to the object as a property. This allows you to use methods of one object as generic utility functions.

You can also use any kind of object which is ArrayLike as the second parameter. In practice, this means that it needs to have a length property, and integer ("index") properties in the range (0..length - 1). For example, you could use a NodeList, or a custom object like { 'length': 2, '0': 'eat', '1': 'bananas' }. You can also use arguments.

// min / max number in an array
let max = fnApply(Math.max, null, [ 21, 42, 84, 168, 7, 3 ]);
// 168

let min = fnApply(Math.min, null, [ 21, 42, 84, 168, 7, 3 ]);
// 3

const module1 = {
    prefix: "Hello",
    x: 21,
    getX() {
        return this.x;
    },
    log(value: string) {
        return this.prefix + " " + value + " : " + this.x
    }
};

// The 'this' parameter of 'getX' is bound to 'module'.
module1.getX(); // 21
module1.log("Darkness"); // Hello Darkness : 21

// Create a new function 'boundGetX' with the 'this' parameter bound to 'module'.
let module2 = {
    prefix: "my",
    x: 42
};

// Call the function of module1 with module2 as it's this
fnApply(module1.getX, module2); // 42
fnApply(module1.log, module2, [ "friend" ]); // my friend : 42
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