javascript经典面试题整理

水深无声 2022-01-23 03:45 234阅读 0赞

一.类型转换
    
    var a = false + 1;
    
    console.log(a); //1
    
    
    
    var b = false == 1;
    
    console.log(b);//false
    
    
    
    if(typeof(a) && (-true) + (+undefined) + ''){
    
    console.log('通过了');
    
    }
    
    // typeof(a) --> 'undefined'
    
    // -true --> -1 +undefined --> NaN
    
    // -1 + NaN-->NaN NaN + '' --> 'NaN'
    
    
    
    if(1+5*'3' === 16){
    
    console.log('通过了');
    
    }
    
    
    
    console.log(!!' ' + !!'' + !!false ||'未通过');//1
    
    
    
    window.a || (window.a = '1');
    
    //()优先级高，先给a赋值，然后进行判断，window.a=='1',为true返回
    
    二.
    
    var fn = (
    
    function test1(){
    
    return '1';
    
    },
    
    function test2(){
    
    return '2';
    
    }
    
    )();
    
    console.log(typeof(fn));//string
    
    
    
    var a = 10;
    
    if(function b(){}){//不是false,所以为true
    
    a += typeof b;//(function b(){})函数声明被()包含生成表达式，b被忽略
    
    }
    
    console.log(a);//'10undefined'
    
    三.
    
    var name = 'ddd';
    
    name += 10;
    
    var type = typeof name;
    
    //如果想要输出string var type = new String(typeof name);
    
    if(type.length === 6){
    
    type.text = 'string';
    
    }
    
    console.log(type.text);//undefined
    
    
    
    var x = 1,
    
    y = z = 0;
    
    function add(n){
    
    return n = n + 1;
    
    }
    
    y = add(x);
    
    function add(n){//预编译，将前面的add覆盖
    
    return n = n + 3;
    
    }
    
    z = add(x);
    
    console.log(x,y,z);//1,4,4
    
    
    
    
    function test(x,y,a){
    
    a = 10;
    
    console.log(arguments[2]);//10 映射关系
    
    arguments[1] = 9;
    
    console.log(y);//9
    
    }
    
    test(1,2,3);
    
    四.
    
    var name = '222';
    
    var a = {
    
    name:'111',
    
    say:function(){
    
    console.log(this.name);
    
    }
    
    }
    
    var fun = a.say;
    
    //var fun = fuunction(){console.log(this.name)} this指向window
    
    fun();//222
    
    a.say();//111 对象里面的方法，this指向a
    
    var b = {
    
    name:'333',
    
    say:function(fun){
    
    fun();
    
    }
    
    }
    
    b.say(a.say);//222 this指向window
    
    b.say=a.say;
    
    // b.say=fuunction(){console.log(this.name)}
    
    b.say();//333
    
    
    
    function Foo(){
    
    getName = function(){//未经过var的变量会提升到全局，会把之前的getName覆盖
    
    console.log(1);
    
    }
    
    return this;
    
    }
    
    Foo.getName = function(){
    
    console.log(2);
    
    }
    
    Foo.prototype.getName = function(){
    
    console.log(3);
    
    }
    
    var getName = function(){
    
    console.log(4);
    
    }
    
    function getName(){
    
    console.log(5);
    
    }
    
    Foo.getName();//2
    
    getName();//4
    
    Foo().getName();//1
    
    getName();//1
    
    new Foo.getName();//2 .运算优先级高于new
    
    new Foo().getName();//3 new Foo()先执行
    
    new new Foo().getName();//3