[[VarNames]],

What Is It Good For?

"Well, I think absolutely nothing."

– Shu-yu Guo (Google), Feb Apr 2024 TC39

What is [[VarNames]]

So,

• In general, we disallow lexical and var bindings to share the same name in the same scope
• The global scope is special
• It's open, unlike a function scope. You can have multiple <script> tags processed at different times adding things to the same global scope
• vars are also properties on globalThis

What is [[VarNames]]

<script>

var x;

</script>

<script>

let x;

</script>

This is disallowed.

​

Fine. Good. Whatever.

What is [[VarNames]]

<script>

Object.defineProperty(

globalThis, 'x',

{ value:42, configurable:false });

</script>

<script>

let x;

</script>

This is also disallowed.

​

…Okay. Sure. Seems fine.

What is [[VarNames]]

<script>

eval('var x');

</script>

<script>

let x;

</script>

This is also disallowed.

​

Seems fine, except…

how do you implement it?

Remember the direct eval var semantics

• Adds a delete-able binding
• When at the global scope, adds a property to globalThis
• Upshot: adds a configurable property to globalThis
• But wait, this is allowed:

​

<script>

globalThis.x = 42;

</script>

<script>

let x;

</script>

[[VarNames]] distinguishes eval-introduced vars

So [[VarNames]] is a list on the global environment whose sole purpose is to distinguish sloppy-direct-eval-introduced vars from ordinary configurable properties

​

• Annoying to understand
• Annoying to implement
• Complexity in service of no one: what are the use cases here?

Are there actual use cases?

• You shouldn't be using sloppy direct eval to introduce global vars
• You can redeclare sloppy direct eval introduced global vars: you just have to delete them first!

Current semantics

In the global scope,

• It is a SyntaxError to redeclare a let or const with a like-named let or const
• It is a SyntaxError to redeclare a non-configurable property with a like-named let or const
• It is a SyntaxError to declare a let or const with a name present in [[VarNames]]

Proposal

In the global scope,

• It is a SyntaxError to redeclare a let or const with a like-named let or const
• It is a SyntaxError to redeclare a non-configurable property with a like-named let or const
• It is a SyntaxError to declare a let or const with a name present in [[VarNames]]

Upshot

<script>

eval('var x');

</script>

<script>

let x;

</script>

This is now allowed.

​

let x will shadow.

Updates from last time

• Moved from PR into proposal at https://github.com/tc39/proposal-redeclarable-global-eval-vars

​

Stage 2.7?