getPrototypeOf()

Summary

The getPrototypeOf() trap "returns the prototype (i.e. the value of the internal [[Prototype]] property) of the specified object."

Examples

In JavaScript, it's very common to have a constructor function with methods defined on the prototype property of the function:

function Rectangle(width, height) {
  this.width = width;
  this.height = height;
}
Rectangle.prototype.getArea = function() {
  return this.width * this.height;
};

var r = new Rectangle(3, 4);
r.getArea(); // returns 12

The prototype of r in this case is Rectangle.prototype.

Reflect.getPrototypeOf(r) === Rectangle.prototype;
// returns true

The getArea() method is available to the r object because it inherits from Rectangle.prototype.

Usage in es7-membrane

I do not recommend disabling the getPrototypeOf() trap, because prototypes are central to the normal operation of JavaScript.  Currently, it throws an exception when disabled.

Coming soon

When the getPrototypeOf() trap is disabled, it will return null to indicate there is no prototype available.

Reflect.getPrototypeOf() by Mozilla Contributors is licensed under CC-BY-SA 2.5 .

setPrototypeOf()

Summary

The setPrototypeOf() trap "sets the prototype (i.e., the internal [[Prototype]] property) of a specified object to another object or to null."

Examples

In JavaScript, it's very common to have a constructor function with methods defined on the prototype property of the function:

function Rectangle(width, height) {
  this.width = width;
  this.height = height;
}
Rectangle.prototype.getArea = function() {
  return this.width * this.height;
};

var r = new Rectangle(3, 4);
r.getArea(); // returns 12

The prototype of r in this case is Rectangle.prototype. The getArea() method is available to the r object because it inherits from Rectangle.prototype.

Setting the prototype means altering all the inherited properties of the object you're inspecting:

function Rectangle(width, height) {
  this.width = width;
  this.height = height;
}
Rectangle.prototype.getArea = function() {
  return this.width * this.height;
};

var r = new Rectangle(3, 4);

const newProto = {
  getPerimeter: function() {
    return (this.width + this.height) * 2;
  }
};

Reflect.setPrototypeOf(r, newProto);

r.getPerimeter(); // returns 14;

try {
  r.getArea(); // throws TypeError: r.getArea is not a function
}
catch (e) {
  // do nothing
}
  
Reflect.setPrototypeOf(newProto, Rectangle.prototype);
  
r.getArea(); // returns 12

Usage in es7-membrane

I strongly recommend disabling the setPrototypeOf() trap, because anyone who sets a prototype on your objects effectively owns all the inherited properties of your objects. For this reason, the setPrototypeOf() trap is not checked by default.

Reflect.setPrototypeOf() by Mozilla Contributors is licensed under CC-BY-SA 2.5 .