JavaScript类型判断

Posted 2020-11-27 Blue Waters

整理一下js里面的类型判断

<script>
let boolType = true;
let numberType = 1;
let stringType = ‘abc‘;
let undefinedType = undefined;
let nullType = null;
let arrayType = [1,2,3,4];
let objectType = {name:‘xiaoming‘,age:22};
let functionType = function(){console.log(‘hello‘)};
let symbolType = Symbol();
let nanType = NaN;
/***
操作符typeof
不能识别null，Array，都把它统一归为object类型
***/
console.log(‘-----------------typeof-----begin-------------------‘)
console.log(‘bool‘,typeof boolType); //boolean
console.log(‘number‘,typeof numberType);//number
console.log(‘string‘,typeof stringType);//string
console.log(‘undefined‘,typeof undefinedType);//undefined
console.log(‘null‘,typeof nullType);//object
console.log(‘array‘,typeof arrayType);//object
console.log(‘object‘,typeof objectType);//object
console.log(‘function‘,typeof functionType);//function
console.log(‘symbol‘,typeof symbolType); //symbol
console.log(‘nan‘,typeof nanType); //number
console.log(‘-----------------typeof-----end---------------------‘)
/***
instanceof
通过原型链判断
不能识别出基本的数据类型 number、boolean、string、undefined、unll、Symbol
***/
console.log(‘-----------------instanceof-----begin-------------------‘)
console.log(‘bool‘,boolType instanceof Boolean); 
console.log(‘number‘,numberType instanceof Number);
console.log(‘string‘,stringType instanceof String);
console.log(‘undefined‘,undefinedType instanceof Object);
console.log(‘null‘,nullType instanceof Object);
console.log(‘array‘,arrayType instanceof Array);
console.log(‘object‘,objectType instanceof Object);
console.log(‘function‘,functionType instanceof Function);
console.log(‘symbol‘,symbolType instanceof Symbol);
console.log(‘nan‘,nanType instanceof Number);
console.log(‘-----------------instanceof-----end---------------------‘)
/***
constructor
返回对创建此对象的函数的引用
但是可以人为的创建一个对象，更改它的原型
null、undefined没有construstor方法，因此constructor不能判断undefined和null
***/
console.log(‘-----------------constructor-----begin-------------------‘)
console.log(‘bool‘,boolType.constructor === Boolean);// true
console.log(‘number‘,numberType.constructor === Number);// true
console.log(‘string‘,stringType.constructor === String);// true
console.log(‘array‘,arrayType.constructor === Array);// true
console.log(‘object‘,objectType.constructor === Object);// true
console.log(‘function‘,functionType.constructor === Function);// true
console.log(‘symbol‘,symbolType.constructor === Symbol);//true
console.log(‘nan‘,symbolType.constructor === Number);//false
console.log(‘-----------------constructor-----end---------------------‘)
/*** 最推荐的方式
Object.prototype.toString.call()
运算符用来测试一个对象在其原型链中是否存在一个构造函数的 prototype 属性
***/
console.log(‘-----------------Object.prototype.toString.call()-----begin-------------------‘)
console.log(‘bool‘,Object.prototype.toString.call(boolType));//[object Boolean]
console.log(‘number‘,Object.prototype.toString.call(numberType));//[object Number]
console.log(‘string‘,Object.prototype.toString.call(stringType));//[object String]
console.log(‘undefined‘,Object.prototype.toString.call(undefinedType));//[object Undefined]
console.log(‘null‘,Object.prototype.toString.call(nullType));//[object Null]
console.log(‘array‘,Object.prototype.toString.call(arrayType));//[object Array]
console.log(‘object‘,Object.prototype.toString.call(objectType));//[object Object]
console.log(‘function‘,Object.prototype.toString.call(functionType));//[object Function]
console.log(‘symbol‘,Object.prototype.toString.call(symbolType)); //[object Symbol]
console.log(‘nan‘,Object.prototype.toString.call(nanType)); //[object Number]
console.log(‘-----------------Object.prototype.toString.call()-----end---------------------‘)
/***
NaN是个比较特别的存在怎么判断是NaN
***/
console.log(‘nan‘,Object.prototype.toString.call(nanType) == ‘[object Number]‘ && nanType != nanType);// true