JavaScript: object and prototype

When writing a construct function, the default properties it owns including:

• prototype
• arguments
• name
• length
• contractor
• apply, bind, call, caller
• hasOwnProperty, isPropertyOf, toString, valueOf…

So does an object have an prototype property? It has not. Instead of ‘prototype’, it has the secret ’__proto__’ property to related to its contractor’s prototype object. The following is some of my tests:

var o = {}
o.constructor // function Object() { [native code] }
o.constructor.prototype //Object {}
o.constructor.prototype.constructor //function Object() { [native code] }
o.constructor.prototype.constructor.prototype //Object {}

var i, hasOwn = Object.prototype.hasOwnProperty;

for (i in o) { if (hasOwn.call(o, i)) { console.log(i+ ":"+ o[i]); }}
//undefined
o.name='abc';
for (i in o) { if (hasOwn.call(o, i)) { console.log(i+ ":"+ o[i]); }}
// name:abc
for (i in o) { console.log(i+ ":"+ o[i]); } //name:abc

o.toString()
"[object Object]"
o instanceof Object  // true
typeof o  //"object"

o.__proto__ // Object {}
o.constructor.prototype === o.__proto__ // true

instanceof vs typeof

the ’instanceof’ and ’typeof’ are 2 ways used to judge variables, here is the difference I got from stackoverflow:

• Use instanceof for custom types
  obj instanceof Person
• Use typeof for simple built in types
  typeof variable === ‘undefined’ typeof variable === ‘number’ typeof variable === ‘string’
• Use instanceof for complex built in types