Javascript中的typeof和instanceof
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typeof 是一元操作符，而instanceof是二元操作符；
typeof 操作的是一个变量，而instanceof前面是一个变量，后面是一个类型；
typeof 返回的是一个字符串，而instanceof 返回的是一个布尔值。
1、typeof()
http://www.cnblogs.com/jikey/archive/2010/05/05/1728337.html typeof 是一个一元运算，放在一个运算数之前，运算数可以是任意类型。 示例 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html;charset=utf-8"> <title>javascript测试</title> <script type="text/javascript"> document.write ("typeof(1): "+typeof(1)+"<br />"); document.write ("typeof(NaN): "+typeof(NaN)+"<br />"); document.write ("typeof(Number.MIN_VALUE): "+typeof(Number.MIN_VALUE)+"<br />"); document.write ("typeof(Infinity): "+typeof(Infinity)+"<br />"); document.write ("typeof(\"123\"): "+typeof("123")+"<br />"); document.write ("typeof(true): "+typeof(true)+"<br />"); document.write ("typeof(window): "+typeof(window)+"<br />"); document.write ("typeof(Array()): "+typeof(new Array())+"<br />"); document.write ("typeof(function(){}): "+typeof(function(){})+"<br />"); document.write ("typeof(document): "+typeof(document)+"<br />"); document.write ("typeof(null): "+typeof(null)+"<br />"); document.write ("typeof(eval): "+typeof(eval)+"<br />"); document.write ("typeof(Date): "+typeof(Date)+"<br />"); document.write ("typeof(sss): "+typeof(sss)+"<br />"); document.write ("typeof(undefined): "+typeof(undefined)+"<br />") </script> </head> <body> </body> </html> 结果
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typeofhttps://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof The
示例 // Numbers
typeof 37 === ‘number‘;
typeof 3.14 === ‘number‘;
typeof(42) === ‘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!
// Strings
typeof "" === ‘string‘;
typeof "bla" === ‘string‘;
typeof (typeof 1) === ‘string‘; // typeof always returns a string
typeof String("abc") === ‘string‘; // but never use this form!
// Booleans
typeof true === ‘boolean‘;
typeof false === ‘boolean‘;
typeof Boolean(true) === ‘boolean‘; // but never use this form!
// Symbols
typeof Symbol() === ‘symbol‘
typeof Symbol(‘foo‘) === ‘symbol‘
typeof Symbol.iterator === ‘symbol‘
// Undefined
typeof undefined === ‘undefined‘;
typeof declaredButUndefinedVariable === ‘undefined‘;
typeof undeclaredVariable === ‘undefined‘;
// Objects
typeof {a:1} === ‘object‘;
// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [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‘;
// Functions
typeof function(){} === ‘function‘;
typeof class C {} === ‘function‘;
typeof Math.sin === ‘function‘;
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2、instanceof
instanceofhttps://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof The
The // defining constructors
function C(){}
function D(){}
var o = new C();
alert(o instanceof C);// true, because: Object.getPrototypeOf(o) === C.prototype
alert(o instanceof D);// false, because D.prototype is nowhere in o‘s prototype chain
alert(o instanceof Object);// true
alert(C.prototype instanceof Object);// true
C.prototype = {};
var o2 = new C();
alert(o2 instanceof C);//true
alert(o instanceof C);// false, because C.prototype is nowhere in o‘s prototype chain anymore
D.prototype = new C();// use inheritance
var o3 = new D();
alert(o3 instanceof D);//true
alert(o3 instanceof C);//true |
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