javascript - JavaScript 使用 let 和 var 关键字时的意外行为

Question

当我使用 var 关键字声明变量时，JS 引擎会在创建阶段为“message”分配默认值

console.log(message); //undefined       
var message = "My message";

但是使用let关键字

console.log(message); //message is not defined       
let message = "My message";

为什么会出现这种意想不到的结果或者是 ES6 中发生了一些变化？

Answer 1

这是因为 Temporal dead zone 的创建与 let

let bindings are created at the top of the (block) scope containing the declaration, commonly referred to as "hoisting". Unlike variables declared with var, which will start with the value undefined, let variables are not initialized until their definition is evaluated. Accessing the variable before the initialization results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the initialization is processed.