JavaScript Proxy : Object

constructor

The Proxy allows you to wrap an Object (called the target) and modify the behavior of that Object.

ECMAScript 2015 Spec

Constructors

new Proxy(target : Object, handler : ProxyHandler) : Proxy

Creates a new Proxy for the target where handler allows you to modify the behavior of target.

Example:

// Create a Proxy for Map that only allows even number keys 
var evenMap = new Proxy(new Map(), {
  // The 'get' function allows you to modify the value returned
  // when accessing properties on the proxy
  get: function(target, name) {
    // Return a custom function for Map.set that checks
    // that the keys are even numbers
    if (name === 'set') {
      return function(key, value) {
        if (Number.isInteger(key) && (key % 2 === 0)) {
          return target.set(key, value);
        }
        else {
          throw Error('key "' + key + '" is not even');
        }
      };
    }
    else {
      var value = target[name];

      // If the value is a function, return a function that
      // is bound to the original target. Otherwise the function
      // would be called with the Proxy as 'this' and Map
      // functions do not work unless the 'this' is the Map.
      if (value instanceof Function) {
        return value.bind(target);
      }

      // Return the normal property value for everything else
      return value;
    }
  }
});

evenMap.set(0, 'foo');
console.log(evenMap.get(0));

evenMap.set(2, 'bar');
console.log(evenMap.get(2));

try
{
  evenMap.set(3, 'baz');
  console.log(evenMap.get(3));
}
catch (e)
{
  console.log(e);
}

Run

Results:

Spec

Proxy Methods

revocable(target : Object, handler : Proxyhandler) : Proxy

Spec

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