typeof and Instanceof in JavaScript

typeof is a unary operator, while Instanceof is a two-dollar operator;

typeof operation is a variable, and instanceof is preceded by a variable, followed by a type;

typeof returns a string, and Instanceof returns a Boolean value.

1, typeof ()

Http://www.cnblogs.com/jikey/archive/2010/05/05/1728337.html TypeOf is a unary operation that can be any type before it is placed in an operand. The return value is a string that describes the type of the operand. , typeof generally can only return the following results:number,boolean,string,function,object,undefined. Example <! doctype html public  "-//w3c//dtd html 4.01 transitional//en"   "HTTP// Www.w3.org/TR/html4/loose.dtd "> Results 650) this.width=650; "src=" Http://s1.51cto.com/wyfs02/M01/86/9C/wKiom1fFN0DAytq7AABAdtNGTTc316.png "title=" 005. PNG "alt=" Wkiom1ffn0daytq7aabadtngttc316.png "/>

typeof Https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof the typeof operator Returns a String indicating the type of the unevaluated operand. 650) this.width=650; "src=" Http://s4.51cto.com/wyfs02/M00/86/9C/wKioL1fFP9fTtJFqAAAyPkwLkX0389.png "title=" 007. PNG "alt=" Wkiol1ffp9fttjfqaaaypkwlkx0389.png "/> 650) this.width=650; "src=" Http://s1.51cto.com/wyfs02/M02/86/9C/wKiom1fFQAHg1d1gAACEybsqp0g657.png "title=" 008. PNG "alt=" Wkiom1ffqahg1d1gaaceybsqp0g657.png "/> Example  Numberstypeof 37 ===  ' number ';typeof 3.14 ===  ' number '; typeof  = ==  ' number ';typeof math.ln2 ===  ' number ';typeof infinity ===  ' number '; typeof nan ===  ' number '; // despite being  "Not-a-number" Typeof Number ( 1)  ===  ' number ';  // but never use this form!// stringstypeof   ""  ===  ' string ';typeof  "Bla"  ===  ' string ';typeof  (typeof 1)  ===   ' string ';  // typeof always returns a stringtypeof string ("abc")   ===  ' string ';  // but never use this form!// booleanstypeof true  ===  ' Boolean ';typeof false ===  ' Boolean '; Typeof boolean (true)  ===  ' Boolean ';  // but never use this form!// symbolstypeof symbol ()  == =  ' symbol ' Typeof&nbsP Symbol (' foo ')  ===  ' symbol ' typeof symbol.iterator ===  ' symbol '// undefinedtypeof  undefined ===  ' undefined ';typeof declaredbutundefinedvariable ===  ' undefined '; typeof undeclaredvariable ===  ' undefined '; // objectstypeof {a:1} ===  ' Object ';// use array.isarray or object.prototype.tostring.call// to  differentiate regular objects from arraystypeof [1, 2, 4] ===  ' Object '; typeof new date ()  ===  ' object ';// the following is confusing.  don ' T use!typeof new boolean (True)  ===  ' object '; typeof new  Number (1)  ===  ' object ';  typeof new string ("abc")  ===  ' object ';//  Functionstypeof function () {} ===  ' function ';typeof class c {} ===  ' function ';typeof math.sin ===  ' function '; null This stands since the beginning of javascripttypeof null = = = ' object '; in the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects is 0. typeof  return value. (reference) A Fix was proposed to ECMAScript (via an opt-in), but is rejected. It would has resulted in typeof null === ‘null‘ .

2, instanceof

instanceof Https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof the instanceof operatortests whether an object have theprototype property of A constructor in its prototype chain. 650) this.width=650; "src=" Http://s1.51cto.com/wyfs02/M01/86/9C/wKiom1fFO7WxIihTAAAo22TeoZQ424.png "title=" 006. PNG "alt=" Wkiom1ffo7wxiihtaaao22teozq424.png "/> theinstanceofoperator tests presence ofconstructor.prototypeinchobject' s prototype chain. Defining Constructorsfunction C () {}function D () {}var o = new C (); alert (o instanceof c);/true, Because:Object.getProt Otypeof (o) = = = C.prototypealert (o instanceof D);//False, because D.prototype is nowhere in O ' s prototype Chainalert (o INS Tanceof object);//Truealert (C.prototype instanceof object);//Truec.prototype = {};var O2 = new C (); alert (O2 instanceof C );//truealert (o instanceof c);//False, because C.prototype is nowhere in O ' s prototype chain anymored.prototype = new C () ;//Use Inheritancevar O3 = new D (); alert (O3 instanceof D);//truealert (O3 instanceof C);//true

typeof and Instanceof in JavaScript