Parser option to allow new.target outside functions

[overlookmotel]

Q	A
Fixed Issues?	n/a
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	Yes
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#2724
Any Dependency Changes?	No
License	MIT

This PR adds an option for parse to allow new.target outside of functions.

There are 2 places this can be useful:

1. In CommonJS code, new.target at top level is legal (if fairly pointless) as CommonJS code is compiled in a function wrapper.
2. When parsing code which will be wrapped in a function or execute in eval() where it inherits new.target from the outer scope.

I've called the option allowNewTargetOutsideFunction to match allowAwaitOutsideFunction and allowReturnOutsideFunction.

If accepted, I'll make a 2nd PR on the website repo to document this new option.

[liuxingbaoyu]

LGTM, we generally need two approves and see what other people think.

[fisker]

I think this will make Prettier able to format this

[fisker]

Maybe allowNewTargetOutsideClass? Bad idea, class is also a function.

[liuxingbaoyu]

@fisker
I test this in the babel repl without errors, and also the prettier doesn't use recoverable exceptions?

UPDATED: It seems that prettier is parsing some of the statements.

[fisker]

That example is a "range format" feature. Only take few lines to format.

[fisker]

I just realize, it seems we are doing things wrong, we should not re-parse the string in range, should directly print the node in range.

[liuxingbaoyu]

Personally it seems to me that it is normal to parse only part of it (eg pasting inside vscode), but this exception should be ignored by prettier.

[nicolo-ribaudo]

This seems ok. I would eventually like to replace all these allow* options with something like context: "async function", or context: "class method", etc.

We just released 7.20.0, so it will be a while before the 7.21.0 (which will include this PR).

@fisker You have to parse the whole file (and just print the specific section) regardless of this, otherwise you cannot properly format this example: https://prettier.io/playground/#N4Igxg9gdgLgprEAuEAzArlMMCW0BUABKgBQCUhwAOlIYQIyEDUhJAnjnADYAmRAzGRoBfEABoQEAA65oAZ2SgAhgCcVEAO4AFVQgUolXDUrYKJAIxVKwAazgwAykoC2cADI4ocZKkNy4Fla29g5S1p4A5sgwKugBIHDO5nA8PCluSlAR6EoRcABiECrOSjC4WcggSugwEOIgABYwzlwA6g048HJhYHAOep04AG6dbJVgcmYgnv4qMFpWESU+fvEAVnIAHg6RXHAAiugQ8Ctc-hJhKrOV5krJXPVSKp4wrTg8MA3IABwADBfqfytKxSSpPOCzIbeCQARyO8AW0n0VTkAFovCkUvUVHA4TgcQtcsskL4zvF-M4cNFYuTdgd4d4SasJDA7m8Pl8kAAmFlWHBcSIAYQgzmJCTkAFZ6uh-AAVO76UnnEBDOIASSgaVgDjAzxkAEFNQ4YGw9qdlVYsn1WXNkPRvhJLXkAKKa5D8LnCYRAA

[fisker]

Good example, I'll try to fix that later.

[overlookmotel]

Thanks for the review. What's the process for submitting PR to update docs? Do it now, wait until this PR is merged, or wait until 7.21.0 is ready for release?

[liuxingbaoyu]

Usually now, we'll merge this PR together when it's released.

[JLHwung]

The current implementation also allows new.target within an arrow function. If it is desired, can you update the note and add a new test?

[overlookmotel]

To clarify: Are you saying that "top level" here is the wrong words to use because inside an arrow function is not top level?

[nicolo-ribaudo]

Given that

function fn() {
  () => new.target
} 

is valid, I would expect () => new.target to be valid with this option, right?

[JLHwung]

I don't think "top level" is wrong. But maybe it is incomplete in this case, since () => new.target will also be allowed.

[overlookmotel]

Yes, intent with this option was to allow new.target anywhere. If the file was wrapped in a function e.g. if it's a CommonJS file, I'm not aware of anywhere where new.target can't be used - because there's always a function above it.

Have added tests for new.target in an arrow function and updated the description of the option accordingly.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/53955/