箭头函数打印 var 但不打印 let 变量

Question

我有这个代码

var key = "outer value";
let obj = {
  key:"obj value",
  func:()=> this.key,
}

console.log(obj.func());

但是把var改成let的时候，结果是undefined。请指导。

Answer 1

let key = "outer value";
let obj = {
  key:"obj value",
  func:()=> key,
}

console.log(obj.func()); //--> Outer Value


var key = "outer value";
let obj = {
  key:"obj value",
  func:()=> key,
}

console.log(obj.func()); //--> Outer Value

即使不使用这个关键字，答案也是一样的

var key = "outer value";
let obj = {
  key:"obj value",
  func:()=> key,
}

console.log(obj.func());

Answer 2

箭头函数没有 its on this，它总是使用父 this

您的代码有效，因为密钥是全局范围的(this)

let key = "outer value";
let obj = {
  key:"obj value",
  func:()=> key,
}
console.log(obj.func());

这行不通，因为键是局部作用域，不在其中

function test() {
  var key = "outer value";
  let obj = {
    key: "obj value",
    func: () => this.key,
  };

  console.log(obj.func()); // undefined
}
test();

这会起作用

function test() {
  this.key = "outer value";
  let obj = {
    key: "obj value",
    func: () => this.key,
  };

  console.log(obj.func()); // outer value
}
test();

箭头函数打印 var 但不打印 let 变量

Arrow Function this prints var but does not print let variable

javascript