JS面试题（面试必看）

  alert(typeof [1, 2]); //object 
    alert(typeof 'leipeng'); //string 
    var i = true;  
    alert(typeof i); //boolean 
    alert(typeof 1); //number 
    var a;  
    alert(typeof a); //undefined
    function a(){};
    alert(typeof a) //function

var btn = document.getElementById("hello");
btn.onclick = function(){
alert(1);
}
btn.onclick = function(){
alert(2);
}

var btn = document.getElementById("hello");
btn.addEventListener("click",function(){
alert(1);
},false);
btn.addEventListener("click",function(){
alert(2);
},false);

var ev = ev || window.event 
document.documentElement.clientWidth || document.body.clientWidth 
var target = ev.srcElement||ev.target

function A( age, name ){ 
this.age = age;
this.name = name;
A.prototype.show = function(){
alert('父级方法');
}
function B(age,name,job){
A.apply( this, arguments );
this.job = job;
B.prototype = new A();
var b = new A(14,'侠客行');
var a = new B(15,'狼侠','侠客');

canceBubble()只支持IE,return false,stopPropagation()

obj.appendChid() 
obj.insertBefore()
obj.replaceChild()
obj.removeChild()

if(typeof Array.isArray==="undefined")
{
  Array.isArray = function(arg){
        return Object.prototype.toString.call(arg)==="[object Array]"
    }; 
}

document.getElementById(“ID”).value

var domList = document.getElementsByTagName(‘input’) 
var checkBoxList = []; 
var len = domList.length;　　//缓存到局部变量 
while (len--) {　　//使用while的效率会比for循环更高 
if (domList[len].type == ‘checkbox’) { 
    　　checkBoxList.push(domList[len]); 
} 
}

var dom = document.getElementById(“ID”); 
dom.innerHTML = “xxxx”
dom.style.color = “#000”

var undefined; 
undefined == null; // true 
1 == true;   // true 
2 == true;   // false 
0 == false;  // true 
0 == '';     // true 
NaN == NaN;  // false 
[] == false; // true 
[] == ![];   // true

var a = new Object();
a.value = 1;
b = a;
b.value = 2;
alert(a.value);

function combo(msg){
    var arr=msg.split("-");
    for(var i=1;i<arr.length;i++){
        arr[i]=arr[i].charAt(0).toUpperCase()+arr[i].substr(1,arr[i].length-1);
    }
    msg=arr.join("");
    return msg;
}

function escapeHtml(str) { 
return str.replace(/[<>”&]/g, function(match) {
    switch (match) {
                   case “<”:
                      return “<”;
                   case “>”:
                      return “>”;
                   case “&”:
                      return “&”;
                   case “\””:
                      return “"”;
      }
  });
}

var foo = 1; 
(function(){
    console.log(foo);
    var foo = 2;
    console.log(foo);
})()
答案：输出undefined 和 2。上面代码相当于：
var foo = 1;
(function(){
    var foo;
    console.log(foo); //undefined
    foo = 2;
    console.log(foo); // 2; 
  })()

function randomNub(aArray, len, min, max) {
  if (len >= (max - min)) {
  return '超过' + min + '-' + max + '之间的个数范围' + (max - min - 1) + '个的总数';
  }
  if (aArray.length >= len) {
  aArray.sort(function(a, b) {
  return a - b
  });
  return aArray;
  }
  var nowNub = parseInt(Math.random() * (max - min - 1)) + (min + 1);
  for (var j = 0; j < aArray.length; j++) {
  if (nowNub == aArray[j]) {
  randomNub(aArray, len, min, max);
  return;
  }
  }
  aArray.push(nowNub);
  randomNub(aArray, len, min, max);
  return aArray;
  }
  var arr=[];
  randomNub(arr,10,10,100);

var array1 = ['a','b','c'];
var bArray = ['d','e','f'];
var cArray = array1.concat(bArray);
cArray.splice(1,1);

createDocumentFragment()    //创建一个DOM片段
createElement()   //创建一个具体的元素
createTextNode()   //创建一个文本节点

appendChild()      //添加
removeChild()      //移除
replaceChild()      //替换
insertBefore()      //插入

getElementsByTagName()    //通过标签名称
getElementsByName()     //通过元素的Name属性的值
getElementById()        //通过元素Id，唯一性

function serilizeUrl(url) { 
var urlObject = {}; 
if (/\?/.test(url)) { 
var urlString = url.substring(url.indexOf("?") + 1); 
var urlArray = urlString.split("&"); 
for (var i = 0, len = urlArray.length; i < len; i++) { 
var urlItem = urlArray[i]; 
var item = urlItem.split("="); 
urlObject[item[0]] = item[1]; 
} 
return urlObject; 
} 
return null; }

var regMail = /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+((.[a-zA-Z0-9_-]{2,3}){1,2})$/;

for(var i=1;i<=3;i++){ 
  setTimeout(function(){
      console.log(i);  
    },0); 
};

for(var i=1;i<=3;i++){
   setTimeout((function(a){  //改成立即执行函数
       console.log(a);   
   })(i),0); 
};

if (!String.prototype.trim) {
 String.prototype.trim = function() {
 return this.replace(/^\s+/, "").replace(/\s+$/,"");
 }
}
  // test the function
var str = " \t\n test string ".trim();
alert(str == "test string"); // alerts "true"