Briefly talk about type judgment in Javascript_javascript skills-JS Tutorial-php.cn

There are several ways to determine the data type

1. Unary operator typeOf

2. Relational operator instanceof

3. constructor attribute

4. prototype attribute

1. typeof

The return values of typeof are as follows

Type	Structure
Undefined	`"undefined"`
Null	`"object"` (see below)
Boolean value	`"boolean"`
Value	`"number"`
String	`"string"`
Symbol (new in ECMAScript 6)	`"symbol"`
Host object (provided by JS environment, such as browser)	Implementation-dependent
Function object (implements [[Call]] in ECMA-262 terms)	`"function"`
Any other object	`"object"`

Simple and crude method, just look at the code

// 以下代码在版本 Google Chrome 45.0.2454.101 m 中测试通过
// Numbers
console.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!
 
// Strings
console.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!
 
// Booleans
console.log(typeof true === 'boolean');
console.log(typeof false === 'boolean');
console.log(typeof Boolean(true) === 'boolean'); // 不要这样使用!
 
// Symbols
console.log(typeof Symbol() === 'symbol');
console.log(typeof Symbol('foo') === 'symbol');
console.log(typeof Symbol.iterator === 'symbol');
 
// Undefined
console.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');

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typeof can only check the above 7 types

2. instanceof

The instanceof operator is used to identify the type of object being processed, requiring developers to explicitly confirm that the object is a specific type

1. instanceof has nothing to do with constructor

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

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2. Instanceof is also called the relational operator, which can be used to determine whether the prototype attribute of a certain constructor exists on the prototype chain of another object to be detected

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

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Why does the third output return false? Original address: A question about instanceof in Javascript

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

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Let’s look at another complicated usage

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

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Why, why is this? You need to understand the following meaning

1. How is this operator defined in the language specification?

2. JavaScript prototypal inheritance mechanism

Object instanceof Object

// 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

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

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Function instanceof Function

FunctionL = Function, FunctionR = Function;
console.log(FunctionL instanceof FunctionR);//true
console.log(FunctionL.__proto__ === Function.prototype); //true

<strong>Foo instanceof Foo
</strong>
function Foo(){}
var foo = new Foo();
FooL = Foo, FooR = Foo;
console.log(FooL instanceof FooR);//false
console.log(FooL.__proto__ === Function.prototype );//true
console.log(FooL.__proto__.__proto__ === Object.prototype );//true
console.log(FooL.__proto__.__proto__.__proto__ === null );//true

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Application of instanceof in Dojo inheritance mechanism

In JavaScript, there is no concept of multiple inheritance, just like Java. But when using declare to declare a class in Dojo, it is allowed to inherit from multiple classes

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

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instanceof and multiple global objects (interaction between multiple frames or multiple windows)

In a browser, our script may need to interact between multiple windows. Multiple windows mean multiple global environments, and different global environments have different global objects and thus different built-in type constructors. This may cause some problems. For example, the expression [] instanceof window.frames[0].Array will return false because Array.prototype !== window.frames[0].Array.prototype, so you must use Array.isArray(myObj) or Object. prototype.toString.call(myObj) === "[object Array]" to determine whether myObj is an array.

// 以下代码在版本 Google Chrome 45.0.2454.101 m 中测试通过
// Numbers
console.log(37 instanceof Number);//false
console.log( 3.14 instanceof Number);.//false
console.log( Math.LN2 instanceof Number);//false
console.log( Infinity instanceof Number);//false
console.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!
 
// Strings
console.log( "" instanceof String);// false
console.log( "bla" instanceof String);// false
console.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!
 
// Booleans
console.log( true instanceof Boolean);// false
console.log( false instanceof Boolean);// false
console.log( Boolean(true) instanceof Boolean); //false 不要这样使用!
 
// Symbols
console.log( Symbol() instanceof Symbol);// false
console.log( Symbol("foo") instanceof Symbol);// false
console.log( Symbol.iterator instanceof Symbol);// false
 
// Undefined
var blabla;
//console.log( undefined instanceof Undefined);// Uncaught ReferenceError: Undefined is not defined
//console.log( blabla instanceof Undefined); // Uncaught ReferenceError: Undefined is not defined
console.log( undefined instanceof Object);// false
console.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);//true
console.log( [1, 2, 4] instanceof Object);//true
console.log( /^[a-zA-Z]{5,20}$/ instanceof Object);//true
console.log( {name:'wenzi', age:25} instanceof Object);//true
console.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);//true
console.log( new Number(1) instanceof Object);//true
console.log( new Date() instanceof Object);//true
console.log( new String("abc") instanceof Object);//true
console.log( new Error() instanceof Object);//true
 
// 函数
console.log( function(){} instanceof Function );//true
console.log( Math.sin instanceof Function);//true

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Note: undefined and null are detected Object types, because there are no global types of Undefined and Null in js, and number, string and boolean cannot detect their types

3. constructor

When using instanceof to detect variable types, we cannot detect the types of number, 'string', and bool. Therefore, we need to find another way to solve this problem

Object.prototype.constructor returns a reference to the function that created the prototype of the object. Note that the value of this property is the function itself, not a string containing the function name. For primitive values (such as 1, true or "test"), this property is read-only, and all objects inherit a constructor property from its prototype

Constructor is originally a property on the prototype object, pointing to the constructor. However, according to the order in which the instance object searches for attributes, if there is no instance attribute or method on the instance object, it will be searched on the prototype chain. Therefore, the instance object can also use the constructor attribute

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

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However, please note that the constructor attribute can be modified, which will cause the detected results to be incorrect

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

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Change the value of the constructor property of this object

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,
  /(&#63;:)/,
  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") );

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Except undefined and null, the type of other types of variables can be determined using constructor

4. The universal Object.prototype.toString.call

Use toString() method to detect object type

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

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Attached is the judgment function. How much do you know about the data types in Javascript

5. Implementation of jquery jquery: "1.8.2",

jquery provides a $.type interface, take a look at the code

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 &#63; 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
},

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It can be seen that jquery is implemented using Object.prototype.toString.call