javascript中typeof和instanceof

Posted 2020-09-26 _linka

typeof：返回值是一个字符串，一般只能返回以下结果

number、string、object、boolean、undefined、function、symbol

Type	Result
Undefined	"undefined"
Null	"object" (see below)
Boolean	"boolean"
Number	"number"
String	"string"
Symbol (new in ECMAScript 2015)	"symbol"
Host object (provided by the JS environment)	Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms)	"function"
Any other object	"object"

  1 // Numbers
  2 typeof 37 === \'number\';
  3 typeof 3.14 === \'number\';
  4 typeof(42) === \'number\';
  5 typeof Math.LN2 === \'number\';
  6 typeof Infinity === \'number\';
  7 typeof NaN === \'number\'; // Despite being "Not-A-Number"
  8 typeof Number(1) === \'number\'; // but never use this form!
  9 
 10 
 11 // Strings
 12 typeof \'\' === \'string\';
 13 typeof \'bla\' === \'string\';
 14 typeof \'1\' === \'string\'; // note that a number within a string is still typeof string
 15 typeof (typeof 1) === \'string\'; // typeof always returns a string
 16 typeof String(\'abc\') === \'string\'; // but never use this form!
 17 
 18 
 19 // Booleans
 20 typeof true === \'boolean\';
 21 typeof false === \'boolean\';
 22 typeof Boolean(true) === \'boolean\'; // but never use this form!
 23 
 24 
 25 // Symbols
 26 typeof Symbol() === \'symbol\'
 27 typeof Symbol(\'foo\') === \'symbol\'
 28 typeof Symbol.iterator === \'symbol\'
 29 
 30 
 31 // Undefined
 32 typeof undefined === \'undefined\';
 33 typeof declaredButUndefinedVariable === \'undefined\';
 34 typeof undeclaredVariable === \'undefined\';
 35 
 36 
 37 // Objects
 38 typeof {a: 1} === \'object\';
 39 
 40 // use Array.isArray or Object.prototype.toString.call
 41 // to differentiate regular objects from arrays
 42 typeof [1, 2, 4] === \'object\';
 43 
 44 typeof new Date() === \'object\';
 45 
 46 
 47 // The following is confusing. Don\'t use!
 48 typeof new Boolean(true) === \'object\';
 49 typeof new Number(1) === \'object\';
 50 typeof new String(\'abc\') === \'object\';
 51 
 52 
 53 // Functions
 54 typeof function() {} === \'function\';
 55 typeof class C {} === \'function\';
 56 typeof Math.sin === \'function\';
 57 
 58 
 59 // This stands since the beginning of javascript
 60 typeof null === \'object\';

instanceof

object instanceof constructor (object: 要检测的对象 constructor: 某个构造函数 )

instanceof 运算符用来检测 constructor.prototype 是否存在于参数 object 的原型链上。

  1 // 定义构造函数
  2 function C(){}
  3 function D(){}
  4 
  5 var o = new C();
  6 
  7 // true，因为 Object.getPrototypeOf(o) === C.prototype
  8 o instanceof C;
  9 
 10 // false，因为 D.prototype不在o的原型链上
 11 o instanceof D;
 12 
 13 o instanceof Object; // true,因为Object.prototype.isPrototypeOf(o)返回true
 14 C.prototype instanceof Object // true,同上
 15 
 16 C.prototype = {};
 17 var o2 = new C();
 18 
 19 o2 instanceof C; // true
 20 
 21 o instanceof C; // false,C.prototype指向了一个空对象,这个空对象不在o的原型链上.
 22 
 23 D.prototype = new C(); // 继承
 24 var o3 = new D();
 25 o3 instanceof D; // true
 26 o3 instanceof C; // true

  1 var simpleStr = "This is a simple string";
  2 var myString  = new String();
  3 var newStr    = new String("String created with constructor");
  4 var myDate    = new Date();
  5 var myObj     = {};
  6 
  7 simpleStr instanceof String; // returns false, 检查原型链会找到 undefined
  8 myString  instanceof String; // returns true
  9 newStr    instanceof String; // returns true
 10 myString  instanceof Object; // returns true
 11 
 12 myObj instanceof Object;    // returns true, despite an undefined prototype
 13 ({})  instanceof Object;    // returns true, 同上
 14 
 15 myString instanceof Date;   // returns false
 16 
 17 myDate instanceof Date;     // returns true
 18 myDate instanceof Object;   // returns true
 19 myDate instanceof String;   // returns false