How does instanceof work in JavaScript? -

in following code sample both checks of obj2 , obj3 @ end instanceof return true if ways there constructed different , results of returning name property different.

var obj1 = function() {     this.name = "foo1"; }; obj1.prototype.name = "foo1onprot"; var obj1 = new obj1();  var obj2 = function() {}; obj2.prototype = new obj1(); obj2.prototype.constructor = obj2; var obj2 = new obj2();  var obj3 = function() {}; obj3.prototype = object.create(obj1.prototype); obj3.prototype.constructor = obj3; var obj3 = new obj3();  console.dir(obj1); console.log("obj1.name: " + obj1.name);  console.dir(obj2); console.log("obj2.name: " + obj2.name);  console.dir(obj3); console.log("obj3.name: " + obj3.name);  console.log("obj2 instanceof obj1: " + (obj2 instanceof obj1)); console.log("obj3 instanceof obj1: " + (obj3 instanceof obj1)); 

result of run in chrome:

obj1   name: "foo1"   __proto__: object     constructor: function () {     name: "foo1onprot"     __proto__: object obj1.name: foo1 obj2   __proto__: obj1     constructor: function () {}     name: "foo1"     __proto__: object       constructor: function () {       name: "foo1onprot"       __proto__: object obj2.name: foo1 obj3    __proto__: object    constructor: function () {}    __proto__: object      constructor: function () {      name: "foo1onprot"      __proto__: object obj3.name: foo1onprot  obj2 instanceof obj1: true obj3 instanceof obj1: true 

what best way recognize obj2 , obj3 different? how instanceof work?

most simply: obj instanceof constructor yields true when obj has constructor's prototype in it's constructor/prototype chain. in other words, asking engine whether obj can treated instance of constructor / whether obj behaves constructor object.

there small handful of syntaxes allow put constructor's prototype in obj's prototype chain. , of them cause obj instanceof constructor true. in examples, both obj2 , obj3 have obj1 in prototype chain.

so, when ask javascript engine whether either obj2 or obj3 behave instance of obj1, javascript assumes true -- case wherein wouldn't if you've overridden obj1's behavior down line.