Lexical declaration cannot appear in a single-statement context

代碼1

if (true) {
    var x = 1;
}

代碼2

if (true)
    var x = 1;

代碼3

if (true) {
    let x = 1;
}

代碼4

if (true)
    var x = 1;

對於上述四段代碼，代碼4會報錯 “Lexical declaration cannot appear in a single-statement context”，對此 TypeScript 的作者 Anders Hejlsberg 說:

JavaScript only permits let and const declarations within a block

Github Issue

有人評論說：

https://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations
let and const declarations define variables that are scoped to the running execution context’s LexicalEnvironment.
https://www.ecma-international.org/ecma-262/6.0/#sec-lexical-environments
A Lexical Environment consists of an Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical Environment is associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement and a new Lexical Environment is created each time such code is evaluated.
https://www.ecma-international.org/ecma-262/6.0/#sec-declarative-environment-records
Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations.

想了想，對於 JavaScript 引擎來講，完全可以把這種情況當做是在塊作用域內聲明變量，但這並無意義，因為這種情況下單行使用 let 聲明變量，必然沒有使用到這個變量的地方，如果有，那就必須使用大括號顯式地表明是塊作用域。所以在阮一峰老師的《ECMAScript6 入門》一書中有一句：

如果沒有大括號，JavaScript 引擎就認為不存在塊級作用域。