简单谈谈Javascript中类型的判断

// 以下代码在版本 Google Chrome 45.0.2454.101 m 中测试通过// Numbersconsole.log(typeof 37 === 'number');console.log(typeof 3.14 === 'number');console.log(typeof Math.LN2 === 'number');console.log(typeof Infinity === 'number');console.log(typeof NaN === 'number'); // 尽管NaN是"Not-A-Number"的缩写,意思是"不是一个数字"console.log(typeof Number(1) === 'number'); // 不要这样使用! // Stringsconsole.log(typeof "" === 'string');console.log(typeof "bla" === 'string');console.log(typeof (typeof 1) === 'string'); // console.log(typeof返回的肯定是一个字符串console.log(typeof String("abc") === 'string'); // 不要这样使用! // Booleansconsole.log(typeof true === 'boolean');console.log(typeof false === 'boolean');console.log(typeof Boolean(true) === 'boolean'); // 不要这样使用! // Symbolsconsole.log(typeof Symbol() === 'symbol');console.log(typeof Symbol('foo') === 'symbol');console.log(typeof Symbol.iterator === 'symbol'); // Undefinedconsole.log(typeof undefined === 'undefined');console.log(typeof blabla === 'undefined'); // 一个未定义的变量,或者一个定义了却未赋初值的变量 // Objects 使用Array.isArray或者Object.prototype.toString.call方法可以从基本的对象中区分出数组类型console.log(typeof {a:1} === 'object');console.log(typeof [1, 2, 4] === 'object');console.log(typeof /^[a-zA-Z]{5,20}$/ === 'object');console.log(typeof {name:'wenzi', age:25} === 'object');console.log(typeof null === 'object');//true // 下面的容易令人迷惑，不要这样使用！console.log(typeof new Boolean(true) === 'object');console.log(typeof new Number(1) === 'object');console.log(typeof new Date() === 'object');console.log(typeof new String("abc") === 'object');console.log(typeof new Error() === 'object'); // 函数console.log(typeof function(){} === 'function');console.log(typeof Math.sin === 'function');

var A = function() {};A.prototype = {}; var B = {};console.log(A.constructor);//function Function() { [native code] }console.log(B.constructor);//function Object() { [native code] } var a = new A();A.prototype = {}; var b = new A();b.constructor = A.constructor; console.log(a.constructor === A);//falseconsole.log(a.constructor);//function Object() { [native code] }console.log(typeof A);//function Object() { [native code] } console.log(a.constructor === b.constructor);//falseconsole.log(b.constructor);//function Function() { [native code] } console.log(a instanceof A);//falseconsole.log(b instanceof A);//true

var str = new String("hello world");console.log(str instanceof String);//trueconsole.log(String instanceof Function);//trueconsole.log(str instanceof Function);//false

//表达式一的指向console.log(str.__proto__ === String.prototype);//trueconsole.log(str instanceof String); //true //表达式二的指向console.log(String .__proto__ === Function.prototype);//trueconsole.log(String instanceof Function);//true //表达式三的指向console.log(str .__proto__ === String.prototype);//trueconsole.log(str .__proto__.__proto__ === String.prototype.__proto__);//trueconsole.log(str .__proto__.__proto__ === Object.prototype);//trueconsole.log(str .__proto__.__proto__.__proto__ === null);//trueconsole.log(str instanceof Object);//trueconsole.log(str instanceof Function);//false

console.log(Object instanceof Object);//trueconsole.log(Function instanceof Function);//trueconsole.log(Number instanceof Number);//falseconsole.log(String instanceof String);//false console.log(Function instanceof Object);//true console.log(Foo instanceof Function);//trueconsole.log(Foo instanceof Foo);//false

// 为了方便表述，首先区分左侧表达式和右侧表达式ObjectL = Object, ObjectR = Object;console.log(ObjectL instanceof ObjectR);//true

// 下面根据规范逐步推演console.log(ObjectL.__proto__ === Function.prototype); //trueconsole.log(ObjectL.__proto__.__proto__ === Object.prototype);//true

FunctionL = Function, FunctionR = Function;console.log(FunctionL instanceof FunctionR);//trueconsole.log(FunctionL.__proto__ === Function.prototype); //trueFoo instanceof Foofunction Foo(){}var foo = new Foo();FooL = Foo, FooR = Foo;console.log(FooL instanceof FooR);//falseconsole.log(FooL.__proto__ === Function.prototype );//trueconsole.log(FooL.__proto__.__proto__ === Object.prototype );//trueconsole.log(FooL.__proto__.__proto__.__proto__ === null );//true

dojo.declare("Aoo",null,{});dojo.declare("Boo",null,{});dojo.declare("Foo",[Aoo,Boo],{}); var foo = new Foo();console.log(foo instanceof Aoo);//trueconsole.log(foo instanceof Boo);//false console.log(foo.isInstanceOf(Aoo));//trueconsole.log(foo.isInstanceOf(Boo));//true

// 以下代码在版本 Google Chrome 45.0.2454.101 m 中测试通过// Numbersconsole.log(37 instanceof Number);//falseconsole.log( 3.14 instanceof Number);.//falseconsole.log( Math.LN2 instanceof Number);//falseconsole.log( Infinity instanceof Number);//falseconsole.log( NaN instanceof Number); // false尽管NaN是"Not-A-Number"的缩写,意思是"不是一个数字"console.log( Number(1) instanceof Number); // false不要这样使用! // Stringsconsole.log( "" instanceof String);// falseconsole.log( "bla" instanceof String);// falseconsole.log( ( 1) instanceof String); // falseconsole.log(返回的肯定是一个字符串console.log( String("abc") instanceof String); // false 不要这样使用! // Booleansconsole.log( true instanceof Boolean);// falseconsole.log( false instanceof Boolean);// falseconsole.log( Boolean(true) instanceof Boolean); //false 不要这样使用! // Symbolsconsole.log( Symbol() instanceof Symbol);// falseconsole.log( Symbol("foo") instanceof Symbol);// falseconsole.log( Symbol.iterator instanceof Symbol);// false // Undefinedvar blabla;//console.log( undefined instanceof Undefined);// Uncaught ReferenceError: Undefined is not defined//console.log( blabla instanceof Undefined); // Uncaught ReferenceError: Undefined is not definedconsole.log( undefined instanceof Object);// falseconsole.log( blabla instanceof Object);// false // Objects 使用Array.isArray或者Object.prototype.toString.call方法可以从基本的对象中区分出数组类型console.log( {a:1} instanceof Object);//trueconsole.log( [1, 2, 4] instanceof Object);//trueconsole.log( /^[a-zA-Z]{5,20}$/ instanceof Object);//trueconsole.log( {name:'wenzi', age:25} instanceof Object);//trueconsole.log( null === Object);//false // 下面的容易令人迷惑，不要这样使用！console.log( new Boolean(true) instanceof Object);//trueconsole.log( new Number(1) instanceof Object);//trueconsole.log( new Date() instanceof Object);//trueconsole.log( new String("abc") instanceof Object);//trueconsole.log( new Error() instanceof Object);//true // 函数console.log( function(){} instanceof Function );//trueconsole.log( Math.sin instanceof Function);//true

function Person(){ }var Tom = new Person(); console.log(Tom.constructor === Person);//true

function Person(){ }function Student(){ }Student.prototype = new Person();var John = new Student();console.log(John.constructor==Student); // falseconsole.log(John.constructor==Person); // true

function Type() { }; var types = [  new Array,  [],  new Boolean,  true,    // remains unchanged  new Date,  new Error,  new Function,  function(){},  Math,   new Number,  1,      // remains unchanged  new Object,  {},  new RegExp,  /(?:)/,  new String,  "test"    // remains unchanged]; for(var i = 0; i < types.length; i++) {  types[i].constructor = Type;  types[i] = [ types[i].constructor, types[i] instanceof Type, types[i].toString() ];}; console.log( types.join("/n") );

function Type() { }; var toString = Object.prototype.toString;console.log(toString.call(new Date) === '[object Date]');//trueconsole.log(toString.call(new String) ==='[object String]');//trueconsole.log(toString.call(new Function) ==='[object Function]');//trueconsole.log(toString.call(Type) ==='[object Function]');//trueconsole.log(toString.call('str') ==='[object String]');//trueconsole.log(toString.call(Math) === '[object Math]');//trueconsole.log(toString.call(true) ==='[object Boolean]');//trueconsole.log(toString.call(/^[a-zA-Z]{5,20}$/) ==='[object RegExp]');//trueconsole.log(toString.call({name:'wenzi', age:25}) ==='[object Object]');//trueconsole.log(toString.call([1, 2, 3, 4]) ==='[object Array]');//true//Since JavaScript 1.8.5console.log(toString.call(undefined) === '[object Undefined]');//trueconsole.log(toString.call(null) === '[object Null]');//true

var m = Object.prototype.toString //501行 E = {};//512行 isFunction: function(a) { //645行  return p.type(a) === "function"},isArray: Array.isArray || function(a) {  return p.type(a) === "array"},isWindow: function(a) {  return a != null && a == a.window},isNumeric: function(a) {  return !isNaN(parseFloat(a)) && isFinite(a)},type: function(a) {  return a == null ? String(a) : E[m.call(a)] || "object"},isPlainObject: function(a) {  if (!a || p.type(a) !== "object" || a.nodeType || p.isWindow(a))    return !1;  try {    if (a.constructor && !n.call(a, "constructor") && !n.call(a.constructor.prototype, "isPrototypeOf"))      return !1  } catch (c) {    return !1  }  var d;  for (d in a)    ;  return d === b || n.call(a, d)},isEmptyObject: function(a) {  var b;  for (b in a)    return !1;  return !0},