ES6 disallowed variable names - why are the names 'let' and 'const' behaving differently?

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I was playing around with unadvisable variable declarations and came to the observations below.

Defining a variable with the name 'let', like so:

let let = 7;

somewhat unsurprisingly leads to the error: SyntaxError: let is disallowed as a lexically bound name.

Attempting to do the same with the variable name 'const', like so:

let const = 7;

leads to the different error: SyntaxError: Unexpected token 'const'.

Looking at the specs, section 13.3.1.1 Static Semantics: Early Errors disallows 'let' as a variable name in this type of declarations (without mentioning 'const').

This is more of a curiosity, but what is happening behind the scenes, which would make 'let' merely be seen as a disallowed variable name while making 'const' be interpreted as a token?

(Incidentally var let = 7 appears to be syntactically correct JavaScript, while var const = 7 also leads to the error SyntaxError: Unexpected token 'const'.)

ecmascript-6 javascript
1 Answers

Basically for backwards compatibility reason. Like Felix said const is a reserved word - for a while now.

Code like the below is legal JavaScript (in loose mode) since let is just a word:

var let = 50;
console.log(let);

Note that indeed when strict mode was added let was reserved inside it:

The following tokens are also considered to be FutureReservedWords when they occur within strict mode code (see 10.1.1).

Reserves let.

As a fun fact const is reserved in the ES3 specification too.

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