Bug: Parser: End range of Program differs from Espree

[overlookmotel]

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• I have tried restarting my IDE and the issue persists.
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Relevant Package

parser

Playground Link

https://typescript-eslint.io/play/#ts=5.7.2&showAST=es&fileType=.tsx&code=PTAEDMHtIKBgPOJQCMCGAnGQ&eslintrc=N4KABGBEBOCuA2BTAzpAXGYBfEWg&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

// foo

x

// bar

ESLint Config

tsconfig

Expected Result

The end location of Program differs between @typescript-eslint/parser and Espree.

Both TS-ESLint and Espree set the start location of Program as the first token, excluding whitespace and comments.

Espree sets the end location as the last token, also excluding whitespace and comments. However, @typescript-eslint/parser does not match that behavior - end location of Program is the end of the file.

I assume that @typescript-eslint/parser is aiming to align with Espree, and therefore this difference is unintentional.

Acorn does not exclude whitespace/comments from either start or end location. It's unfortunate the the 2 most-used ESTree-compatible parsers do not match each others' behavior, and personally I prefer the Acorn way - I think it's surprising that you can have comments which have locations outside the Program which contains them. But I imagine quite possible that neither Acorn nor Espree may want to make a breaking change.

Examples:

• @typescript-eslint/parser
• Espree
• Acorn

Actual Result

See above.

Additional Info

No response

Versions

package	version
@typescript-eslint/parser	8.29.0

[bradzacher]

It sounds to me like espree is wrong here?
Why would the range not include the comments and whitespace?

[overlookmotel]

Why would the range not include the comments and whitespace?

That is my feeling too. I'll raise an issue on Espree and see what they say.

However, at present @typescript-eslint/parser's behavior is half-and-half. It excludes whitespace/comments from the start range, but includes them in the end range. Whichever (include or exclude) is the "right" way to do it, the current behavior seems inconsistent.

[JLHwung]

Here is another case: with ranges option enabled:

// single comment

In espree: program.ranges are [0, 17] while ts-eslint gives [17, 17]

[overlookmotel]

Here is another case: with ranges option enabled:

// single comment

This is a great example! The fact that it's not clear what the "right" answer is suggests there's a problem with the mental model. I'd have expected [0, 0].

Note: Esprima (which Espree aims to follow) is also [17, 17]. No-one agrees on this.

Have opened an issue in Espree's repo: eslint/js#648

[bradzacher]

IMO comments are part of the Program -- whether there is code or not.
So I'd personally say that us reporting [17, 17] is wrong.

What you guys are probably finding as you work on alignment is that when you look at ranges and tokens things get very messy.

The main problem with the ESTree spec is that it solely specifies AST shape and nothing more. So ranges, locs, tokens are all left as an exercise for the parser to guess at.

[overlookmotel]

Yes, that's exactly what we're finding!

Here's another one - the ranges of TemplateElement quasis:

const template = `abc${x}def${x}ghi`;
// Acorn:         ^^^    ^^^    ^^^
// TS-ESLint:    ^^^^^^ ^^^^^^ ^^^^^

In my view, Acorn's behavior is correct here. ${ is not part of the quasi, it's after it!

But TS-ESLint's behavior matches Espree, which matches Esprima.

The broader question is: Do you feel that TS-ESLint's priority should be to align with Espree? Or, if you feel Espree's behavior is incorrect (as you do with the range of Program), are you willing to diverge from Espree to fix what you see as a bug?

[fisker]

I think it's already discussed in espree eslint/js#488