js的常用小技巧

//类对象转成数组

var domNodes = Array.prototype.slice.call(document.getElementsByTagName("*"));   var domNodes = Array.prototype.slice.call(arguments);

向下取整:

var a = ~~1.2; //1

这个补充一下，还可以用位右移符>>

var a = 3.4>>0; //3 

但是两者最好都只用在正整数上，因为只是舍掉了小数部分。Math.floor(-1.2)应该为-2，这两种方法的结果为-1

转数字

var time = + new Date();

设默认值

function foo(bar){
    var foobar = bar || 'default'; 
    //bar 为 undefined, null, "", 0, false, NaN 时最后都得到'default'
}

//坑
[]||"aa"; //[]
{}||"aa"; //SyntaxError
({})||"aa";//Object {}

NaN的坑

//NaN - 不是一个数字
isNaN(a);
//检查是不是 "不是一个数字" ..

isNaN(null);//false
//实际上是null被转为0了,表面上"null 不是一个 不是数字 的东西"。wtf...

UNICODE 用作变量名

var \u4f60\u597d = "\u4f60\u597d";
var b = {};
b.\u4f60\u597d = \u4f60\u597d;
console.log(b);//Object {你好: "你好"}
console.log(b.你好);// "你好"
console.log(b.\u4f60\u597d);// "你好"

console.log("你"==="\u4f60");//true

数组传递和复制

var a = [1,2,3];
var b = a;
delete b[1];
console.log(a);//[1, undefined × 1, 3]

var a = [4,5,6];
var b = a.slice(0);
delete b[1];
console.log(a);//[4,5,6]
console.log(b);//[4, undefined × 1,6]

对象与Function

console.log(typeof Function);//"function"
console.log(typeof Object);//"function"

函数声明

aa();
function aa(){return true;} //true

bb();
var bb = function(){return true;} //TypeError

toString()

2.toString();//SyntaxError
2 .toString(); //"2"
2..toString(); //"2"
(2).toString(); //"2"

[1,[2,"abc","",0,null,undefined,false,NaN],3].toString();
//"1,2,abc,,0,,,false,NaN,3"

for in 暴露原型链属性

Object.prototype.foo = 1;

var obj = new Object();
obj.bar = 1;
for(var p in obj){
    console.log(p);//bar,foo 都遍历出来了。可以用hasOwnProperty()过滤原型链属性

}

switch代替if else

 switch (true) {  
        case (a > 10):  
            do_something();
            break;
        case (a < 100):  
            others();  
            break;  
        default:
            ;  
            break;  
    };  

 

漂亮的随机码

Math.random().toString(16).substring(2); //14位
Math.random().toString(36).substring(2); //11位

合并数组：

var a = [1,2,3];
var b = [4,5,6];
Array.prototype.push.apply(a, b);
console.log(a); //[1,2,3,4,5,6]
用0补全位数：

function prefixInteger(num, length) {
  return (num / Math.pow(10, length)).toFixed(length).substr(2);
}
交换值：

a= [b, b=a][0];

将一个数组插入另一个数组的指定位置：

var a = [1,2,3,7,8,9];
var b = [4,5,6];
var insertIndex = 3;
a.splice.apply(a, Array.concat(insertIndex, 0, b));
// a: 1,2,3,4,5,6,7,8,9

快速取数组最大和最小值

Math.max.apply(Math, [1,2,3]); //3
Math.min.apply(Math, [1,2,3]); //1

条件判断：

var a = b && 1;
//相当于:
if (b) {
  a = 1;
}else {
  a = b;
}

var a = b || 1;
//相当于
if (b) {
  a = b;
} else {
  a = 1;
}

判断数组:

Object.prototype.toString.call([]) === '[object Array]'; 

判断IE:

var ie = /*@cc_on !@*/false;

取整:

a = 5.2|0

string转数字

a = +'123'

4. 去除数组中重复的值：

function removeRepeat(arr){
  return arr.filter(function(elem, pos) {
    return arr.indexOf(elem) == pos;
  });
}

var arr = new Array("1","2","3","4","4","4","4","5");
var newArr = removeRepeat(arr);
console.log(newArr); //1,2,3,4,5

function sum(arr){ return arr.reduce(function(s,n){ return s+n }); } var arr = new Array(1,2,3,4); console.log(sum(arr)); // 10

6. 寻找数组中的最大数

var  numbers = [5, 458 , 120 , -215 , 228 , 400 , 122205, -85411];
var maxNumber = Math.max.apply(null, numbers);
console.log(maxNumber);

判断是否为数组

 function isArray(arr){ return arr ? Object.prototype.toString.call(arr) === '[object Array]' : false }

转数字

var time = + new Date();

• false => ![]
• true => !![]
• undefined => [][[]]
• NaN => +[![]]
• 0 => +[]
• 1 => +!+[]
• 2 => !+[]+!+[]
• 10 => [+!+[]]+[+[]]
• Array => []
• Number => +[]
• String => []+[]
• Boolean => ![]
• Function => []["filter"]
• eval => []["filter"]["constructor"]( CODE )()
• window => []["filter"]["constructor"]("return this")()

http://www.jsfuck.com/