What is an anonymous function in JavaScript? Brief analysis of application scenarios

As the name suggests, anonymous functions refer to functions without names. They are used very frequently in actual development and are also the focus of learning JS well. The following article will give you a detailed introduction to anonymous functions in JavaScript. I hope it will be helpful to you!

##Anonymous function: A function without an actual name.

First we declare a normal function:

//声明一个普通函数，函数的名字叫fn
function fn(){
    console.log("web-chubby");
}

Then remove the name of the function to become an anonymous function:

//匿名函数，咦，运行时，你会发现报错啦！
function (){
    console.log("web-chubby");
}

At this point, you will find When running an anonymous function alone, an error is reported because it does not meet the syntax requirements!

Solution: Just wrap a bracket around the anonymous function to make it an expression:

//匿名函数在其它应用场景括号可以省略
(function (){
    //由于没有执行该匿名函数，所以不会执行匿名函数体内的语句。
    console.log("web-chubby");
})

How to execute and use anonymous functions?

1. Execute anonymous function

If you need to execute an anonymous function, just add a bracket after the anonymous function, that is, execute the function immediately

• The parentheses only wrap the anonymous function followed by the execution parentheses (commonly used)

  
  

(function () {
      alert('匿名函数执行方式一')
})();

• The parentheses will anonymous The function and the parentheses that execute the anonymous function are wrapped together to form an expression

2. Anonymous function parameter passing

is the same as other ordinary parameters. Just write the parameters directly within the brackets:

 (function (m) {
      alert(m)
    }('这是匿名函数传进来的参数'));

Application of anonymous functions

• When binding events Method

<input type="button" value="点我啊！" id="sub">
<script>
    //获得按钮元素
    var sub=document.querySelector("#sub");
    //给按钮增加点击事件。
    sub.onclick=function(){
        alert("当点击按钮时会执行到我哦！");
    }
</script>

• Function expression assigns an anonymous function to a variable

//将匿名函数赋值给变量fn。
var fn=function(){
    return "我是一只小小小小留下，怎么飞也飞不高！"
}
//调用方式与调用普通函数一样
console.log(fn());//我是一只小小小小留下，怎么飞也飞不高！

• object The function attributes inside

var obj={
    name:"web-chubby",
    age:18,
    fn:function(){
        return "我叫"+this.name+"今年"+this.age+"岁了！";
    }
};
console.log(obj.fn());//我叫web-chubby今年18岁了！

• are callback functions, taking the anonymous function as one of the parameters

 //过滤出值为9的值
    let numArr = [1, 5, 9, 10]
    let newArr = numArr.filter(function (item) {
      if (item !== 9) {
        return item
      }
    });

• Function return value, that is, the function is used as a return value

//将匿名函数作为返回值
function fn(){
    //返回匿名函数
    return function(){
        return "web-chubby";
    }
}
//调用匿名函数
console.log(fn()());//web-chubby
//或
var box=fn();
console.log(box());//web-chubby

Imitate block-level scope

Block Level scope, sometimes called private scope. There is no block-level scope in JavaScript. For example:

if(1==1){//条件成立，执行if代码块语句。
    var a=12;//a为全局变量
}
console.log(a);//12
for(var i=0;i<3;i++){
    console.log(i);
}
console.log(i);//4

if(){}for(){}, etc. do not have their own scope. If it goes out of its own scope, the declared variable will be destroyed immediately. But we can simulate block-level scope through anonymous functions:

(function(){
    //这里是我们的块级作用域（私有作用域）
})();

Try block-level scope:

function fn(){
    (function(){
        var la="啦啦啦！";
    })();
    console.log(la);//报错---la is not defined
}
fn();

The role of anonymous functions:

1. Closure can be implemented through anonymous functions. Closure will be explained in the following articles. A quick introduction here: a closure is a function that can access variables defined within the function scope. To create a closure, you often need to use an anonymous function.

2. Simulate block-level scope and reduce global variables. After executing the anonymous function, the corresponding variables stored in the memory will be destroyed, thereby saving memory. Furthermore, in large-scale multi-person development projects, using block-level scope will greatly reduce the problem of naming conflicts, thereby avoiding catastrophic consequences. Developers no longer have to worry about messing up the global scope.

[Related recommendations:

javascript learning tutorial]

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