[读书笔记]高性能JS-编程实践

This part is some example to practice.

1 avoid double evaluation

less use of eval,new Function,settimeout,setinterval.

string in these method are executable js code.when use these method,js will execute code twice ,first is normal js,second is js code in string ,such as var a=eval('1+2');first Asignment,second run code in string '1+2';

in normal,don't use new Function. try to use function instead of string both in serttimeout and setinterval;

2 use literal；

var obj={

　　'a':1,

　　"b":2

}

var arr=[1,2,3,4]

these two are good and run fast than something as 

var obj={};

obj.a=1;

obj.b=2;

3 no repeated job

normally, we addevent use the function as

function addEvent(obj,type,handler){
    if(obj.addEventListener){
        obj.addEventListener(type,handler,false)
    }else{
        obj.attachEvent('on'+type,handler);
    }
}
function removeEvent(obj,type,handler){
    if(obj.removeEventListener){
        obj.removeEventListener(type,handler,false)
    }else{
        obj.detachEvent('on'+type,handler);
    }
}
when called ,function will do repeated job for check which browser we using. actually ,one test is enough.

one way to avoid is lazy load.

function addEvent(obj,type,handler){
    if(obj.addEventListener){
        addEvent= function (obj,type,handler) {
            obj.addEventListener(type,handler,false)
        }
    }else{
       addEvent= function (obj,type,handler) {
           obj.attachEvent('on'+type,handler);
       }
    }
    addEvent(obj,type,handler)
}

call this method once,and new addEvent will override the past one.so next time called,it save lots of time.

and another way is conditional load.

var addevent=document.body.addEventListener?
    function (obj,type,handler) {
        obj.addEventListener(type,handler,false)
    }: function (obj,type,handler){
        obj.attachEvent('on'+type,handler);
    }
cool way.