Node: Assignment to `undefined` and `Infinity`

[19shubham11]

• Version: v12.16.0
• Platform: Darwin Kernel Version 19.0.0: Wed Sep 25 20:18:50 PDT 2019; root:xnu-6153.11.26~2/RELEASE_X86_64 x86_64
• Subsystem: Modules?

What steps will reproduce the bug?

So I created the following script as I was experimenting with strict-mode and found a very weird behaviour. If you execute the following code in a script, it logs 19 twice. In the REPL we get the expected error - Uncaught SyntaxError: Identifier 'undefined' has already been declared, same for Infinity as well.
I'm not sure if this is an expected behaviour for a script or a bug

'use strict'

const undefined = 19
const Infinity = 19

console.log(undefined)
console.log(Infinity)

just save this to a file and run the code to observe the behaviour described above.

What is the expected behavior?

Maybe throw the same error that we see in the REPL?

[mmarchini]

I tested this on d8, jsc and spidermonkey, all threw an error when trying to assign to undefined or Infinity. But this doesn't seem to be a spec violation (at least I couldn't find anything on https://www.ecma-international.org/ecma-262/10.0/ that prevents users from assigning to those identifiers). With that being said, I think it makes sense for us to prevent assigning to those identifiers as well as other common ones, but that will be a breaking change.

[devsnek]

CommonJS is actually a function wrapper!

globalThis.undefined is non-writable non-configurable property, so assigning to it or trying to shadow it at the top level doesn't work, but functions introduce a new scope.

$ eshost -ts -e "(function() { 'use strict'; const undefined = 42; print(undefined) })()"
┌────────────────┬────────────────────────────────────────────┐
│ engine262      │ 42                                         │
│ GraalJS        │ undefined                                  │
│ JavaScriptCore │                                            │
│ SpiderMonkey   │                                            │
│ V8             │                                            │
├────────────────┼────────────────────────────────────────────┤
│ Moddable XS    │ undefined                                  │
│                │ undefined                                  │
├────────────────┼────────────────────────────────────────────┤
│ QuickJS        │                                            │
│                │ SyntaxError: invalid lexical variable name │
└────────────────┴────────────────────────────────────────────┘

You can also do this without a function, just introduce a new lexical scope: { const undefined = 42; print(undefined); }

[bnoordhuis]

I'm closing this as answered.