[Bug]Same function declaration will cause SyntaxError

Bug Report

• I would like to work on a fix!

Current behavior

See this

Input Code

function foo() {}

function foo() {}

Will get SyntaxError: Identifier 'foo' has already been declared error

Expected behavior

No error. Because this is a valid javascript syntax and can be executed without any error thrown

Babel Configuration (babel.config.js, .babelrc, package.json#babel, cli command, .eslintrc)

See REPL above

Environment

REPL

• Babel version(s): REPL
• Node/npm version: REPL
• OS: Windows 10
• Monorepo: no
• How you are using Babel: REPL

Possible Solution

Additional context

[NE-SmallTown]

Our goal is to align to the specification so that when gradually disabling Babel transform over time your code doesn’t suddenly start failing.

That’s fair.

By the way, could we add more info when throw the error? For example, change “SyntaxError: Identifier ‘foo’ has already been declared” to “SyntaxError: Identifier ‘foo’ has already been declared, this is not allowed in a ESM”

[nicolo-ribaudo]

You are saying that it’s possible for Babel to accept invalid code and still produce code that runs.

That’s true, but it’s out of scope for the project. Our goal is to align to the specification so that when gradually disabling Babel transform over time your code doesn’t suddenly start failing.

It’s not ok to silently shallow an error, because that error is mandated by the spec.

That said, you can pass the errorRecovery: true parser option which doesn’t throw for early errors: https://runkit.com/nicolo-ribaudo/babel-babel-12354