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:
这个补充一下,还可以用位右移符>>
设默认值
NaN的坑
UNICODE 用作变量名
数组传递和复制
对象与Function
函数声明
toString()
for in 暴露原型链属性
switch代替if else
var a = ~~1.2; //1
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
条件判断:
判断数组:
判断IE:
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
判断是否为数组var numbers = [5, 458 , 120 , -215 , 228 , 400 , 122205, -85411];
var maxNumber = Math.max.apply(null, numbers);
console.log(maxNumber);
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/