Features of bound functions

bound-functions.js

/**
 * Features of bound functions.
 *
 * See: http://dmitrysoshnikov.com/notes/note-1-ecmascript-bound-functions/
 *
 * by Dmitry Soshnikov <dmitry.soshnikov@gmail.com>
 * MIT Style License
 */

var F = function () {};
F.prototype.y = 20;

var that = {x: 10};
var B = F.bind(that);

// no "prototype" property
console.log("B.prototype:", B.prototype); // undefined

// "F.prototype" property of "F" is used
// to set the "__proto__" of "o"
var o = new B;

console.log("o.y:", o.y); // 20, from "F.prototype"

// and exactly "F.prototype" is used
// in "instanceof" test

console.log("o instanceof B:", o instanceof B); // true
console.log("o instanceof F:", o instanceof F); // true

// manual changing of "B.prototype"
// won't help, still "F.prototype" is used

B.prototype = {
  constructor: B,
  z: 30
};

var q = new B;

console.log("q.z:", q.z); // undefined!, no "B.prototype" is used
console.log("q.y:", q.y); // 20, still from "F.prototype"

// and again
console.log("q instanceof F:", q instanceof F); // true
console.log("q instanceof B:", q instanceof B); // true
console.log("o instanceof F:", o instanceof F); // true
console.log("o instanceof B:", o instanceof B); // true

// But. "Object.create" also sets "__proto__"

var t = Object.create(B.prototype); // use our manual prototype with {... z: 30}

console.log("t.z:", t.z); // 30, huray!, from "B.prototype"

// but...
console.log("t instanceof B:", t instanceof B); // false!, since "F.prototype" is used for test