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Introduction to the use of Javascript high-order functions_javascript skills

WBOYOriginal: 2016-05-16 15:55:001317browse

Higher-order function - If a function receives parameters or returns a function, then we can call this function a higher-order function. As we all know, JavaScript is a weakly typed language: JavaScript functions do not strongly define or type-check input parameters or output values of the function. Then the function can become a parameter or an output value, which embodies the JavaScript Native support for higher-order functions.

1. Higher-order functions whose parameters are functions:

function funcTest(f){  
//简易判断一下实参是否为函数
if((typeof f)==”function”){
f();
}}
funcTest(function(){ });

This is a simple higher-order function that takes parameters as functions. When calling funcTest, input a function as a parameter, and execute the input anonymous function inside funcTest. Of course, such a code snippet has no practical meaning.

1. Higher-order functions whose return values are functions:

function funcTest(){
return function(){};
}
var f=funcTest();

Calling funcTest returns a function.

2. A more complicated example:

//Number类型相加  
function addInt(a,b){
return parseInt(a)+parseInt(b);
 }
//String类型相加
function addString(a,b){
return a.toString()+ b.toString(); 
}  
function add(type){
if(type==="string"){
return addString;
}else{
return addInt; 
}
}
var data1=add("string")("1","2");
//12
var data2=add("int")("1","2");
//3

The above example implements the separation of String type addition and Number type addition. When the add function is called, if the input parameter is "string", a string splicing function is output; if the input parameter is "int", a digital addition function is output.

3. The actual role of higher-order functions:

The above code example basically explains what higher-order functions are. Let’s take a look at how higher-order functions relate to our actual programming:

1, callback function

function callback(value){
alert(value);
}
function funcTest(value,f)
//f实参检测，检查f是否为函数 
if(typeof callback==='function'){
f(value);}}funcTest(‘1',callback);
//1

The example is when funcTest is called, the callback function will be called internally in funcTest, that is, the callback is implemented.

2, Data screening and sorting algorithm

var arr=[0,2,11,9,7,5];
//排序算法
function funcComp(a,b){
if(a<b){
return -1;
}else if(a>b){
return 1;
}else{
return 0;
}
}
//过滤算法
function funcFilter(item,index,array){
return item>=5;
}
//数组顺序排列
arr.sort(funcComp);
alert(arr.join(','));
//0,2,5,7,9,11
//筛选数组
var arrFilter=arr.filter(funcFilter);
alert(arr.join(‘,'))
//5,7,9,11

3, DOM element event definition

<html><title></title>
<body><input type=”button” value=”ClickMe” id=”myBtn” >
<script type=”text/javascript>
var btnClick=document.getElementById(“myBtn”);
//测试环境为FireFox
btnClick. addEventListener(“click”,function(e){
alert(“I'm a button!”);
//I'm a button},false);
</script>
</body>
</html>

In the above example, a button with the id myBtn is defined in the document, and the js script adds a click event to it, where the second parameter of addEventListener is a function.

Conclusion: High-order functions are not a patent of JavaScript, but they are definitely a powerful tool for JavaScript programming. Higher-order functions are actually a re-abstraction of basic algorithms. We can use this to improve the abstraction of the code, achieve maximum code reuse, and write code that is simpler and more conducive to refactoring.

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