CODEWITHSUNDEEP
javascript
lazyboy
lazyboy

Reputation: 311

Why "let" is not listed in the keywords of ECMAScript 2019 (ES10)?

In ECMAScript 2019 one can use let to declare a new (lexical binding) variable. (Specification section 13.3.1 https://www.ecma-international.org/ecma-262/10.0/index.html#prod-LexicalDeclaration )

Why does the list of keywords not contain let ( https://www.ecma-international.org/ecma-262/10.0/index.html#prod-Keyword also in section 11.6.2.1)?

Upvotes: 2

Views: 221

Answers (1)

georg
georg

Reputation: 215029

The list of Javascript reserved words was fixed back in 2000 in the 3rd edition of the spec (page 14). For backwards compatibility it's not possible to extend this list, because that would break existing programs. However, the spec authors couldn't predict how the language will evolve back then, and which new keywords will be needed. As a result, some "newer" keywords are not reserved, unless the strict mode is used.

var let = 1; // valid in the non-strict mode

Technically this means that let, await, yield etc are not "tokens" for the compiler but just identifiers, which are given special meaning only in specific syntactic positions and treated as is otherwise (again, in the non-strict mode):

function *yield() { // identifier
    yield 1; // keyword
}

var let = 1; // identifier
let x = 2; // keyword

The strict mode treats "old" and "new" reserved words equally, although error messages are different ("unexpected token" vs "strict mode reserved word").

Upvotes: 4

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