Javascript：何时何时不使用“this” [英] Javascript: When and when not to use "this"

问题描述

我很好奇，什么时候需要/最好的做法是使用关键字 this 。我知道 this 用于确定函数 this 的值，但它总是需要吗？

我之所以问这个问题，是因为我有一个内部函数，它在我的模块中被调用，它真正做的就是对传递它的一些数据进行排序。我的问题是我应该使用这个关键字来调用这个函数，或者单独使用。

例如：

  function formatSomeData（data）{
 //代码........ 
} 
 
 this.formatSomeData（data）; 
 
或
 
 formatSomeData（data）; 
  

我得到函数被调用的上下文以及它的目的在回答问题，但在这种情况下，就像我提到的那样，我真的不需要在任何时候访问 this 对象。在调用函数时使用它仍然是一个好习惯吗？
我所要求的并不是这个如何工作，而是什么时候适合使用它，何时不适合。

解决方案

何时需要/最佳实践使用关键字this

当你想要访问某些内容时通常会使用它。所以举个例子，如果你有一个自定义对象，并且想要在某个方法中使用某个属性，你应该使用 this 。

function Person（fname，lname）{this.fname = fname; this.lname = lname; this.fullName = function（）{return this.fname +''+ this.lname; }} var p = new Person（'foo'，'bar'）; console.log（p.fullName（））

如果您在当前构造函数中看到，我创建了一个函数（ fullName ），它需要访问 fname 和 lname 属性是它所属的对象的属性。这是一个地方必须使用。

---

现在声明时，何时使用这个？

构造函数中属于 this 的任何属性都将成为对象的一部分。所以如果你需要一些只能被你访问但不在外面的东西，你可以使用函数而不是这个。

函数Person（fname，lname）{var self =false）;}

---

至于你的代码，我已经在注释中解释了它的工作原理：

在非严格模式下，这个将指向窗口和任何不在函数中的声明都将成为全局作用域的一部分。

但正如@

• 没有声明语句定义的任何变量（ var | let | const
• 任何具有声明语句的变量都将成为全局作用域的一部分。



Im curious when it is needed/best practice to use the keyword this. I understand that this is used when determining a functions this value but is it always needed?

The reason I am asking is because I have a internal function and it is called within my module and all it really does is sort some data you pass it. My question is should I call this function using the this keyword or stand alone.

E.g:

function formatSomeData(data){
  //code........
}

this.formatSomeData(data);

        OR

formatSomeData(data);

I get that the context of where the function is being called and what its purpose is matters in answering the question, but in this case like I mentioned, I really don't need to access the this object at any point. Is it still good practice to use it when calling functions? What I'm asking is not so much as to how "this" works, but when is it appropriate to use it and when isn't it.

解决方案

when it is needed/best practice to use the keyword this

This is usually used when you want to access something in some. So for instance, if you have a custom object and want to use some property inside some method, you should use this.

function Person(fname, lname){
  this.fname = fname;
  this.lname = lname;
  this.fullName = function(){
    return this.fname + ' ' + this.lname;
  }
}

var p = new Person('foo', 'bar');
console.log(p.fullName())

If you see, in current constructor, I created a function(fullName) which needs to access fname and lname properties of the object it is part of. This is a place this must be used.

---

Now while declaration, when to use this?

Any property in a constructor that is a part of this will be a part of the object. So if you need something that is only accessible to you but not outside, you can use function instead of this.

function Person(fname, lname){
  var self = this;
  self.fname = fname;
  self.lname = lname;
  
  // This function is private
  function getFullName() {
    var name = '';
    var seperator = '';
    if (fname) {
      name += self.fname;
      seperator = ' ';
    }
    if (lname) {
      name += seperator + self.lname;
    }
    return name;
  }
  this.fullName = function(){
    return getFullName();
  }
}

var p = new Person('foo', 'bar');
console.log(p.fullName())

---

As for your code, I have already explained in comment why it works:

In non-strict mode, this will point to window and any declaration not in a function will be a part of global scope.

But as rightly pointer by @Jonas W its a bad practice. Why it is bad?

• Any variable defined without declaration statement(var|let|const) will become part of global scope.
• Any variable with declaration statement outside any functions will become part of global scope.

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