为什么我不能使用let，const或var关键字在ES6类中声明变量，但可以直接声明它? [英] Why I can not declare a variable inside an ES6 class by using let, const or var keyword but I can declare it directly?

问题描述

对于以下代码，我想知道ES6类中此行为背后的原因:

For the following code I wanted to know the reason behind this behavior in an ES6 Class:

class One {
    //why the following code is not allowed.
    let check = false; 
    const PI = 3.14;   
    var v = 'Hello';    

    //why the following code is allowed.
    chk = false;       
    Pi = 3.14;         
    vv = "Hi";         
}

我知道我可以像下面那样编写代码，但是我想知道上面代码背后的真正原因.

I know I can write code like below but I wanted to know the real reason behind the above code.

class Sample {
   constructor(x, y) {
      this.x= x;
      this.y= y;
   }
} 

推荐答案

class One {
    //why the following code is not allowed.
    let check = false; 
    const PI = 3.14;   
    var v = 'Hello';    

    //why the following code is allowed.
    chk = false;       
    Pi = 3.14;         
    vv = "Hi";         
}

实际上，这些都不是当前合法的javascript.后者是类字段的示例，当前为阶段3提案，因此最终将是合法的语法.通过转译器，您可以立即使用该语法，并且Transpiler会将代码移到构造函数中.

Actually, neither of those is legal javascript currently. The latter is an example of class fields, which is currently a stage 3 proposal, so it will eventually be legal syntax. With a transpiler, you can use that syntax right now and the transpiler will move the code into the constructor.

class One {
  chk = false;       
  Pi = 3.14;         
  vv = "Hi";         
}

大致成为:

class One {
  constructor() {
    this.chk = false;       
    this.Pi = 3.14;         
    this.vv = "Hi";         
  }
}

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