Support sourceType: "unambiguous" parsing #449
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src/index.js
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| child.type === "ImportDeclaration" || | ||
| child.type === "ExportDeclaration" || | ||
| child.type === "ExportAllDeclaration" || | ||
| child.type === "ExportDefaultDeclaration" |
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An interesting case would be AwaitExpression -- though that probably just throws in the babylon stage instead, before even becoming an AwaitExpression.
await is reserved as a keyword at the top level (so not only in async functions), but only when the source type is module (Note 1 in https://tc39.github.io/ecma262/#prod-AwaitExpression).
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You still get an AwaitExpression either way, I think that part of the spec is more to make like easier for parsers if they want.
In this example, if you use await as an identifier it'll throw and reparse as a script successfully, which I think is good?
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Is it possible to warn if ambiguous code is detected?
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## master #449 +/- ##
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- Coverage 98.26% 97.81% -0.45%
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src/index.js
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| // Rather than try to parse as a script first, we opt to parse as a module and convert back | ||
| // to a script where possible to avoid having to do a full re-parse of the input content. | ||
| if (!hasModuleSyntax(ast)) ast.program.sourceType = "script"; |
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The "correct logic" would be to parse as script first and module later if that fails, but true, it's a lot more likely to get code that parses as both (or module only) as input. Not a lot of people still have code that only parses in sloppy mode, even if they still use sloppy mode semantics.
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Yeah, I think it's as long as the output result is the same, the potential better average performance makes me want to go this route.
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For perf I had the acorn folks dig into proofing out an optimized top level parse.
In reify we also do a source sniff for import or export and if it fails we skip it.
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Will node do the same now? |
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Nope! This is still a best guess. The only thing that's certain is that if |
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I think at this point Node will end up with |
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We moved babylon back into the monorepo at https://github.com/babel/babel/tree/master/packages/babylon. Unfortunately pull requests cannot be migrated between repositories automatically. If this PR is still valid please reopen it there. |
This code is ugly so definitely consider a WIP, but I'm posting for feedback before I go any further. This is the same logic used by Webpack to parse files and will result in files being treated as modules if they contain at least one import/export statement.
My proposed usage of this in Babel itself would be to change Babel's current default
sourceType: "module"tosourceType: path.extname(opts.filename || "") === ".mjs" ? "module" : "script"so that files that Babel can begin toggling module transpilation directly based onsourceType, and can throw errors if import statements are inserted into a CommonJS module for instance. Similarly, this means we can stop addinguse strictat the top of CommonJS files automatically and stop rewritingthistoundefined.With this, users can opt into
sourceType: unambiguousin their.babelrcto tell Babel to essentially guess what type of file it is. This same logic is already implemented in Webpack and a new other loaders.