handler.apply()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.

The handler.apply() method is a trap for the [[Call]] object internal method, which is used by operations such as function calls.

Try it

function sum(a, b) {
  return a + b;
}

const handler = {
  apply(target, thisArg, argumentsList) {
    console.log(`Calculate sum: ${argumentsList}`);
    // Expected output: "Calculate sum: 1,2"

    return target(argumentsList[0], argumentsList[1]) * 10;
  },
};

const proxy = new Proxy(sum, handler);

console.log(sum(1, 2));
// Expected output: 3
console.log(proxy(1, 2));
// Expected output: 30

Syntax

js
new Proxy(target, {
  apply(target, thisArg, argumentsList) {
  }
})

Parameters

The following parameters are passed to the apply() method. this is bound to the handler.

target

The target callable object.

thisArg

The this argument for the call.

argumentsList

An Array containing the arguments passed to the function.

Return value

The apply() method can return any value, representing the return value of the function call.

Description

Interceptions

This trap can intercept these operations:

Function call: proxy(...args)
Function.prototype.apply() and Function.prototype.call()
Reflect.apply()

Or any other operation that invokes the [[Call]] internal method.

Invariants

The proxy's [[Call]] internal method throws a TypeError if the handler definition violates one of the following invariants:

The target must be a callable itself. That is, it must be a function object.

Examples

Trapping a function call

The following code traps a function call.

js
const p = new Proxy(function () {}, {
  apply(target, thisArg, argumentsList) {
    console.log(`called: ${argumentsList}`);
    return argumentsList[0] + argumentsList[1] + argumentsList[2];
  },
});

console.log(p(1, 2, 3)); // "called: 1,2,3"
// 6