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