Why is typeof let === 'undefined'?

Question

Why does typeof let return 'undefined' and is not throwing an SyntaxError instead?

console.log(typeof let);

The unary typeof operator expects an expression. Am I missing something about the let statement?

`let` is not a variable. What exactly are you trying to accomplish? — charlietfl, Apr 28 '18 at 00:44
@charlietfl I think this is more of an academic endeavor moreso than it is a practical one — Andrew, Apr 28 '18 at 00:47
I'm interested in this question. With `console.log(typeof var)` you got an error — Davide, Apr 28 '18 at 00:47
@charlietfl I think the OP knows that. s/he wants to know *why is not throwing an SyntaxError instead?* — Ele, Apr 28 '18 at 00:47
Probably as it is not _yet_ a completely reserved keyword like `var` and `class`. It is only a reserved keyword in strict mode — Patrick Evans, Apr 28 '18 at 00:49
Put `"use strict";` at the beginning and you'll get an error. — Barmar, Apr 28 '18 at 00:50
`var let = true; typeof let === 'boolean'` let is a valid variable — generalhenry, Apr 28 '18 at 00:51
Related: [Using 'let' as a variable name is not throwing any errors in google v8](https://stackoverflow.com/q/40585224/3853934) — Michał Perłakowski, May 03 '18 at 20:21

score 10 · Accepted Answer · edited Apr 28 '18 at 20:53

10

The typeof operator is treating let as a undeclared variable.

See more in MDN docs.

Look at this with an undeclared variable.

console.log(typeof elefromstack)

In strict mode, an error is thrown.

'use strict'
console.log(typeof let);

edited Apr 28 '18 at 20:53

Kristianmitk

answered Apr 28 '18 at 00:55

Ele

1 Answers1