transpiled rest param assumes Array is built-ins

[yortus]

Bug Report

The transpiled code for rest params may have different semantics to the original code. The transpiled code injects a reference to Array, assuming it is the builtin array constructor. However if another identifier called Array is in scope, it will be referenced instead.

Input Code

let Array = [];
let f = (...args) => args;
console.log(f(1, 2, 3));

Current behavior

Running the transpiled code for this example throws "TypeError: Array is not a constructor".

Expected behavior
Running the transpiled code outputs [1, 2, 3].

Environment

• Babel version: v7.10.3
• How you are using Babel: babeljs.io/repl

[babel-bot]

Hey @yortus! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

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[JLHwung]

Currently Babel output assumes the built-ins are not modified in a spec-incompliant way. In your case it is shadowed and Array is not a constructor anymore.

[yortus]

Currently Babel output assumes the built-ins are not modified in a spec-incompliant way.

The builtin is not modified, and there is nothing spec-incompliant in the sample code.

This is simple shadowing by an ordinary variable in lexical scope. Babel is just making a faulty assumption here.

The section added to caveats.md in the linked PR (babel/website#2283) doesn't cover babel's assumptions about simple shadowing. To cover this case and eg #10807, I suggest extending the wording in caveats.md to add babel's assumption that the program will not have any variables in scope with the same name as builtins.

Alternatively, since babel knows the variable names in the program being transpiled, it could issue an error here since the output would be invalid. Or even better, refer to builtins in a way that can't clash with local names.