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Object – Prototype Properties and Methods
Properties Properties Properties
Object.prototype.constructor
Specifies the function that creates an object's prototype. All objects will have a constructor property. Objects created without the explicit use of a constructor function (i.e. the object and array literals) will have a constructor property that points to the Fundamental Object constructor type for that object.
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<!DOCTYPE html><html><body><script>
var o = {};
console.log(o.constructor===Object,o.constructor.name);var o = new Object;
console.log(o.constructor===Object,o.constructor.name);var a = [];
console.log(a.constructor===Array,o.constructor.name);var a = new Array;
console.log(a.constructor===Array,o.constructor.name);var n = new Number(3); console.log(n.constructor===Number,o.constructor.name);// true "Object"
(for each line)function C(){this.x=100;}console.log(C); // function C(){this.x=100;}console.log(C.constructor); // function Function() { [native code] }console.log(C.constructor.name); // Function
</script></body><html>The following shows how to test for {}.
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var o = {};console.log(Object.keys(o).length === 0 &&
o.constructor === Object); // true
</script></body><html>Object.prototype.__proto__
Points to the object which was used as the prototype when the object was instantiated, exposing [[Prototype]] (referto 8.2.8 for more). Its use has been controversial but has been standardized in ECMAScript-2015. It is recommended to use Object.getPrototypeOf() instead.
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<!DOCTYPE html><html><body><script>
var C = function () {};var p = {
a: function () {
console.log('a');
}};C.prototype.__proto__ = p;var o = new C();o.a(); //aconsole.log(C.prototype === o.__proto__); //truevar o = new C();o.__proto__ = p;o.a(); //
aconsole.log(C.prototype === o.__proto__); //false
</script></body><html><!DOCTYPE html><html><body><script>
function test() {}test.prototype.myname = function () {
console.log('Hello');}var a = new test;console.log(a.__proto__ === test.prototype);//truea.myname(); // Hellovar obj = {__proto__: test.prototype};obj.myname(); // Hello
</script></body><html>Value Value Value s s s
Object.prototype.valueOf()
Returns the primitive value of the specified object. This function is called automatically when the object is treated as a primitive which is not a string.
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function N() {this.x=1;}N.prototype.valueOf = function(){this.x++;return this.x;}var n = new N;console.log(n); // N {x: 1}console.log(n+1); // 3console.log(n); // N {x: 2}console.log(n+n); // 7
… 3 + 3 + 1
</script></body><html>Object.prototype.toString()
Returns a string representation of the object. This function is called automatically when the object is treated as a string.
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function C() {}var o = new C;console.log(C+""); // function C() {}console.log(o+""); // [object Object]C.prototype.toString = function a(){return "Haha!";}console.log(C+""); // function C() {}console.log(o+""); // Haha!
</script></body><html>Object.prototype.toLocaleString()
Calls toString().This method is meant to be overridden by derived objects for locale-specific purposes.
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Object.prototype.hasOwnProperty()
Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.
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<!DOCTYPE html><html><body><script>
o = {}; o.a = 10;console.log('a' in o, '__proto__' in o); // true trueconsole.log(o.hasOwnProperty('a'),
o.hasOwnProperty('__proto__')); // true false
</script></body><html><!DOCTYPE html><html><body><script>
function C(){this.a = 1;}C.prototype.b = 2;var o = new C;console.log(o.b); // 2for (let p in o) {
console.log(p);if (o.hasOwnProperty(p))
console.log("ownProperty: "+p);} // a
ownProperty: a
b
</script></body><html>Object.prototype.isPrototypeOf( X )
Returns a boolean indicating whether the object this method is called upon is in the prototype chain of X. Unlike 'intanceof X' which compares against X, .isPrototypeOf(X) compares against X.prototype.
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<!DOCTYPE html><html><body><script>
function A() {}function B() {}function C() {}B.prototype = Object.create(A.prototype);C.prototype = Object.create(B.prototype);var o = new C();console.log(o instanceof C
,
C.prototype.isPrototypeOf(o)); // true trueconsole.log(o instanceof B
,
B.prototype.isPrototypeOf(o)); // true trueconsole.log(o instanceof A
,
A.prototype.isPrototypeOf(o)); // true trueconsole.log(o instanceof Object,
Object.prototype.isPrototypeOf(o)); // true true
</script></body><html>Object.prototype.propertyIsEnumerable()
Returns a boolean indicating if the internal ECMAScript [[Enumerable]] attribute is set, ie. if a property is enumerable by a for...in loop, etc., except properties inherited through the prototype chain.
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<!DOCTYPE html><html><body><script>
var a = [10,20];
a.v=30;console.log(a.propertyIsEnumerable(0)); // trueconsole.log(a.propertyIsEnumerable('v')); // trueconsole.log(a.propertyIsEnumerable('constructor')); // falseconsole.log(a.propertyIsEnumerable('length')); // falseconsole.log(this.propertyIsEnumerable('Math')); // falseconsole.log(Math.propertyIsEnumerable('random'));// false
</script></body><html><!DOCTYPE html><html><body><script>
function A() {
this.property = 'is not enumerable';}A.prototype.m1 = function() {};function B() {
this.m2 = function () { return 'is enumerable'; };}B.prototype = new A;B.prototype.constructor = B;var o = new B;o.x = 99;console.log(o.propertyIsEnumerable('x')); // trueconsole.log(o.propertyIsEnumerable('m2')); // trueconsole.log(o.propertyIsEnumerable('property')); // falseconsole.log(o.propertyIsEnumerable('prototype')); // falseconsole.log(o.propertyIsEnumerable('constructor')); // falseconsole.log(o.propertyIsEnumerable('m1')); // false
</script></body><html>