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How tests work
The babel-compat tests are mostly written as fixtures, similar to the @babel/parser tests. The src/convert.rs test runner looks in the fixtures/ directory for input and expected output files. Input files are parsed into an swc AST and converted to a Babel AST in Rust. Output files are parsed directly into a Babel AST. The two ASTs are then compared, with any differences causing the test to fail.
How to write a test
Step 1: Create a new fixture dir and input file.
mkdir fixtures/my-test
echo "var a = true;" > fixtures/my-test/input.js
Step 2: Generate an output file with the expected Babel AST as JSON. There's a utility script available to help, but you can do this however you like.
# If using the babelgen.js utility, run `npm install` first to get @babel/parser dependency.
node babelgen.js fixtures/my-test/input.js > fixtures/my-test/output.json
Step 3: cargo test should now pick up your new test automatically.
There's a small, insignificant different between the default Babel AST and the converted one causing my test to fail.
This happens a lot with None and Some(false). You'll probably want to add a normalizer function to the Normalizer visitor in src/normalize/mod.rs.
Other random utlities
swcgen.js: Prints the swc AST as JSON.compare.sh: prints the Babel and swc ASTs side-by-side.