Detailed analysis of the use of apply method in js_javascript skills

1. The general approach to object inheritance is to copy: Object.extend The implementation of prototype.js is:

Copy the code The code is as follows: Object.extend = function(destination, source) { for (property in source) { destination[property ] = source[property]; } return destination; } In addition, there is another method, which is: Function.apply (of course using Function. You can also call) The apply method can hijack the method of another object and inherit the properties of another object

Function.apply(obj, args) method can receive two parameters

obj: This object will replace this object in the Function class

args: This is an array, which will be passed as parameters to Function (args-->arguments)

apply demonstration code is as follows:

Copy code The code is as follows: function Person(name,age){ //Define a class, human this.name=name; //Name this.age=age; //Age this.sayhello=function (){alert("hello")}; } function Print(){ //Display class attributes this.funcName="Print"; this.show=function() { var msg=[]; for(var key in this){ if(typeof(this[key])!="function"){ msg.push([key, ":",this[key]].join("")); ; >function Student(name,age,grade,school){ //Student class Person.apply(this,arguments); Print.apply(this,arguments); this.grade=grade; //Grade this.school=school; //School } var p1=new Person("jake",10); p1.sayhello(); var s1= new Student("tom",13,6,"Tsinghua Primary School"); s1.show(); s1.sayhello(); alert(s1.funcName);

The student class originally does not have any methods, but after Person.apply(this,arguments),

He has the sayhello method and all attributes of the Person class. After Print.apply(this,arguments), you automatically get the show() method 2. Use Apply’s parameter array to improve

Function.apply() tips for improving program performance Let’s start with the Math.max() function. Math.max can be followed by any number of parameters, and finally returns the maximum value among all parameters.

For example alert(Math.max(5,8)) //8

alert(Math.max(5,7,9,3,1,6)) //9

But in many cases, we need to find the largest element in the array.

Copy code

The code is as follows: var arr=[5,7,9,1] alert(Math.max(arr)) // This is not possible. Be sure to write like this function getMax(arr){ var arrLen=arr.length; for(var i=0,ret=arr[0];i ret=Math. max(ret,arr[i]); } return ret; }

Writing like this is troublesome and inefficient. If you use apply, look at the code:

Copy the code The code is as follows: function getMax2(arr ){ return Math.max.apply(null,arr); } The two pieces of code achieve the same purpose, but getMax2 is much more elegant, efficient and concise.

Another example is the push method of an array. var arr1=[1,3,4]; var arr2=[3,4,5]; If we want to expand arr2, and then append to arr1 one by one, and finally let arr1=[1,3,4,3,4,5] arr1.push(arr2) obviously will not work. Because doing this will get [1,3,4,[3,4,5]]

We can only use a loop to push one by one (of course you can also use arr1.concat(arr2), but the concat method does not change arr1 itself)

Copy the code The code is as follows: var arrLen=arr2.length for(var i=0;i arr1 .push(arr2[i]); } Since there is Apply, things have become so simple Array.prototype.push.apply(arr1,arr2)