JavaScript type check

JavaScript type check memo

instanceof

check whether an object belongs to a certain class

a = new Number('1'); // create a Number object using Number() constructor
b = Number('1'); // create primitive type object number using Number() function

a instanceof Number
> true // a is Number object
b instanceof Number 
> false // b is primitive type object: number
typeof(a);
> 'object'
typeof(b);
> 'number'

prototype chain :

Number.prototype;
> [Number: 0]
a.__proto__;
> [Number: 0] // Number.prototype
a.__proto__.__proto__;
> {}          // Object.prototype
a.__proto__.__proto__.__proto__;
> null        // no deeper prototype

isPrototypeOf

a instanceof Class can be rephrased as Class.prototype.isPrototypeOf(a)

a = new Number('1');
b = Number('1');
Number.prototype.isPrototypeOf(a)
> true
Number.prototype.isPrototypeOf(b) 
> false

getOwnPropertyNames

Object.getOwnPropertyNames(Object)
[ 'length',
  'name',
  'prototype', // the instance of any Object follows this
  'assign',
  'getOwnPropertyDescriptor',
  'getOwnPropertyDescriptors',
  'getOwnPropertyNames', // use this to check properties
   ...
  'keys',
  'entries',
  'values' ]

> Object.getOwnPropertyNames(Object.prototype)
[ 'constructor',
  '__defineGetter__',
   ...
  'propertyIsEnumerable',
  'toString', // prototype.toString just return '[object <object name>]'
  'valueOf',
  '__proto__',
  'toLocaleString' ]

Object.getOwnPropertyNames(Number)
[ 'length',
  'name',
  'prototype',
  'isFinite',
   ...
  'EPSILON' ]

Object.getOwnPropertyNames(Number.prototype)
[ 'constructor',
   ...
  'toString',
  'valueOf',
  'toLocaleString' ]

prototype

Object.getPrototypeOf(123) // prototype of Number
> [Number: 0]
Object.getPrototypeOf('123') // prototype of String
> [String: '']
Object.getPrototypeOf([1,2,3]) // prototype of Array
> []

Object.prototype.toString.call(new Object())
> '[object Object]'
Object.prototype.toString.call(123)
> '[object Number]'
Object.prototype.toString.call('123')
> '[object String]'
Object.prototype.toString.call([1,2,3])
> '[object Array]'

String.prototype.toString.call('123') // String('123').toString()
> '123'
String.prototype.toString.call(123)
/* TypeError: String.prototype.toString requires that 'this' be a String
     at Number.toString (<anonymous>) */

Number.prototype.toString.call(123) // Number(123).toString()
> '123'
Number.prototype.toString.call('123')
/* TypeError: Number.prototype.toString requires that 'this' be a Number
     at String.toString (<anonymous>) */