js高频经典面试题总结

类型转换问题

　　　　　console.log(null>=0);
        console.log(null<=0);
        console.log(null==0);
        console.log(undefined == 0);
 　　　　console.log([] == ![]);
        console.log(0 == "0");
        console.log(1 == "true");
        console.log(false == "false");
        console.log(false == "0");
        console.log(false == undefined);
        console.log(false == null);
        console.log(null == undefined)；
        console.log(" \t\n " == 0);
        console.log({} == !{});
        console.log([] == false);
        console.log(![] == false);
        console.log([] !==false);
        

 var  a = 0;
        for(var i = 0;i<5;i++){
                if(''){
                        a+=i;
                }else if(Number(undefined)){
                        a = 2+i;
                }else if(Number(null)){
                        a = 3+i;
                }else if(Number({})){
                        a = 4+i;
                }else if(Number([])){
                        a = 5+i;
                }else{
                        a++;
                }
        }
        console.log(a);

数值的理解

 　　　　console.log(0.1+0.2 == 0.3);
        console.log(0.3-0.2 == 0.2-0.1);

数据类型

 function getAge(...args) {
                console.log(typeof args);
        }
        getAge(21);

对象理解

  var obj1 = {a:1, b:2};
        var obj2 = {a:1, b:2};
        console.log(obj1 == obj2);
        console.log(obj1 = obj2);
        console.log(obj1 == obj2);

 let a = 3;
        let b = new Number(3);
        let c = 3;
        console.log(a == b);
        console.log(a === b);
        console.log(b === c);
        console.log(a == c);

function makeCounter() {
                var count = 0
                return function() {
                        return count++
                };
        }
        var counter = makeCounter()
        var counter2 = makeCounter();
        // console.log(counter == counter2);
        console.log( counter() )
        console.log( counter() )
        console.log( counter2() )
        console.log( counter2() )

 var fnArr = [];
        for (var i = 0; i < 10; i ++) {
                fnArr[i] =  function(){
                        return i
                };
        }
        console.log( fnArr[3]() )

 var obj1 = {a:1, b:2};
        var obj2 = {a:1, b:2};
        console.log(obj1 == obj2);
        console.log(obj1 = obj2);
        console.log(obj1 == obj2);

  var fullname = 'David Jones';
var obj ={
        fullname: 'Colin Brown',
        prop:{
                fullname:'Aurelio Deftch',
                getFullname: function(){
                        return this.fullname;
                }
        }
        }
        var test = obj.prop.getFullname;
        console.log(test())
       console.log( obj.prop.getFullname()); 

 function checkAge(data) {
                if (data === { age: 18 }) {
                        console.log("You are an adult!");
                } else if (data == { age: 18 }) {
                        console.log("You are still an adult.");
                } else {
                        console.log(`Hmm.. You don't have an age I guess`);
                }
        }
        checkAge({ age: 18 });

 function test(){
                var n=4399;
                function add(){
                        n++;
                        console.log(n);
                }
                return {n:n,add:add}
        }
        var result=test();  
       var result2=test();        
　　　　console.log(result == result2);
        result.add();
        result.add();
        console.log(result.n);
        result2.add();

this的指向

  const shape = {
                radius: 10,
                diameter() {
                        return this.radius * 2;
                },
                perimeter: () => 2 * Math.PI * this.radius
        };
        shape.diameter();
        shape.perimeter();

同步异步

  for(var i = 0;i<5;i++){
                setTimeout(function () {
                        console.log(i);
                },1000);
        }

作用域

 var funcs = [];
        for(var i = 0;i<10;i++){
                funcs.push(()=>{
                        return i;
                });
        }
        funcs.forEach((func) =>{
                console.log(func());
        });

        var funcs2 = [];
        for(var i = 0;i<10;i++){
                funcs2.push(()=>{
                        return i;
                });
                console.log(funcs2[i]());
        }
        funcs2.forEach((func) =>{
                console.log(func());
        });

      var i = 10;
        function f() {
                i = 20;
                console.log(i);
                for(var i = 0;i<6;i++){
                        console.log(i);
                }
                console.log(this.i);
                console.log(i);
        }
        f();
        console.log(i);

  fn1();
　fn2();
        function fn1(){
                console.log("fn1");
        }
        var fn2 = function(){
                console.log("fn2");
        }
        fn2();

 function Foo(){
                getName = function(){
                        alert(1);
                };
                return this;
        }
        Foo.getName = function(){
                alert(2);
        };
        Foo.prototype.getName = function(){
                alert(3);
        };
        var getName = function(){
                alert(4);
        };
        function getName(){
                alert(5);
        }
        //请写出以下答案
        Foo.getName();
        getName();
        Foo().getName();
        getName();
        new Foo.getName();
        new Foo().getName();
        new new Foo().getName();

 var fn = function(){
                console.log(fn);
        }
        fn();
        var  obj = {
                fn2:function () {
                        console.log(fn2);
                }
        };
        obj.fn2();

变量声明

 console.log(a);
        var a=1;
        console.log(a);
        function a(){alert(2)}
        console.log(a);
        var a=3;
        console.log(a);
        function a(){alert(3)}
        console.log(a);
        console.log(a());

运算符相关

var a = 2,b = 3;

        var c = a+++b;
        console.log(c);
        console.log(a+++b);
        console.log(a+++b);

object和function的关系，理解原型

 console.log(Object instanceof Function);
        console.log(Object instanceof Object);
        console.log(Function instanceof Function);
        console.log(Function instanceof Object);
        console.log(Object.constructor);
        console.log(Function.constructor);
        console.log(Function.prototype);
        console.log(Function.__proto__);
        console.log(Function.__proto__ === Function.prototype );//
        console.log(Function.__proto__ === Object.prototype );//，

        console.log(Object.prototype);
        console.log(Object == Function);
        console.log(Object == Function);

  var foo = {n:3};
        (function (foo) {
                console.log(foo.n);
                foo.n = 3;
                var foo = {n:2};
                console.log(foo.n);
        })(foo);
        console.log(foo.n);

 var obj = {"key":"1","value":"2"};
        var newobj = obj;
        newobj.value+=obj.key;
        console.log(obj.value);

 if(!"a" in window){
                var a = 1;
        }
        console.log(a);

 var one;
        var two = null;
        console.log(one == two,one === two);

 var arr = [{a:0},{},[]];
        arr.forEach((items, index) => {
                items.b = index;
        });
        console.log(arr);

 function foo() {
                console.log("first");
                setTimeout(()=>{
                        console.log("second");
                },5);
        }
      for(let i = 0;i<100;i++){
              foo();
      }

未完，持续更新.........