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JavaScript In Practice: ES6 And Beyond

Defining Proxies

Explore how to define proxies in JavaScript to manage object property access and enhance function behavior. This lesson helps you understand traps like get and apply to control data visibility, validate inputs, and count function calls, including recursive ones, using the Reflect API and proxying techniques.

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We can create a proxy in the following way:

Node.js
let proxy = new Proxy( target, trapObject );

The first argument is target representing the proxied constructor function, class, or object.
The second argument is an object containing traps that are executed once the proxy is used in a specific way.
Let’s put our knowledge into practice by proxying the following target:

Node.js
class Student {
    constructor(first, last, scores) {
        this.firstName = first;
        this.lastName = last;
        this.testScores = scores;
    }
    get average() {
        let average = this.testScores.reduce( 
            (a,b) => a + b, 
            0 
        ) / this.testScores.length;
        return average;
    }
}
let john = new Student( 'John', 'Dwan', [60, 80, 80] );
console.log(john);

We will now define a proxy on the target object john:

Node.js
let johnProxy = new Proxy( john, {
    get: function( target, key, context ) {
        console.log( `john[${key}] was accessed.` );
        // return undefined;
    } 
});

The get ...