Pranesh Ravi Pranesh Ravi - 24 days ago 9
Javascript Question

Using 'let' as a variable name is not throwing any errors in google v8

I was writing some random code in the chrome developer console. For my surprise, chrome let me use

let
as a variable name which is completely wrong as
let
is a reserved keyword. I need to understand why is this happening.

Scenarios:

var const = 78 //throws an error as expected

var function = 46 //throws an error as expected

var let = 56 //didn't throw an error :O

let //prints 56, which is wrong because 'let' is a keyword

let ab = 90

ab //prints 90 as expected


This flaw exists in
node
. But, when I try it in Babel REPL it is throwing an error.

I think this is something to do with Google v8

Answer

A nice write-up of the reasoning behind this can be found in this article by Mohsen Azimi. Here's a quick summary of it.

The following keywords are defined in the JavaScript spec as FutureReservedWord:

implements     interface   let       package    private
protected      public      static    yield

In normal mode, these can be used as variable names without errors; however, in strict mode they are treated as reserved words and will throw the following error:

SyntaxError: Cannot use the reserved word 'let' as a variable name in strict mode.

This is so that pre-ES2015 code doesn't break - if someone had named lots of their variables let in a legacy app, they probably wouldn't be happy if the JS spec suddenly broke everything.