javascript - 在 javascript ECMAScript6 中声明变量 "let"

Question

为什么我不能声明 const 或 var，却可以声明名为 let 的变量。我知道我永远不会那样做，但我只是好奇是否有合理的解释。所以我可以这样做:

var let = 5;
let x = 3;
x + let -> 8

为什么这是可能的？

Answer 1

来自Mozilla Developer Network site在 Future reserved keywords 下:

The following are only reserved when they are found in strict mode code:

其中包括 let。

但是，const 和 var 列在 Reserved keywords as of ECMAScript 6 和 this document 下显示它们最初在 ECMAScript 1 中保留的历史。

Strict Mode

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.

什么是严格模式？

ECMAScript 5's strict mode is a way to opt in to a restricted variant of JavaScript. Strict mode isn't just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do