Preval

A wannabe compiler tool for JS.

This project is intended to end up as some kind of compiler-esque tool for JS builds.

For the time being, it just replaces constant bindings with its constant value. Not very exciting. Possibly bugged.

Usage

Run all tests and automatically update snapshots:

./p u

Test one specific file:

./p f tests/constants/copy.md

Tests

The test cases are written to a markdown file which is interpreted in a paritcular way.

The test files contain the input and output of the test case, verbatim.

You can create a new test file by starting a line with a comment. The rest of the file is assumed to be the input code. Otherwise the test is assumed to adhere to the structure of existing tests.

State

At the time of writing, the code base is really just a bootstrap, partially taken from Mope and Tenko, with some snapshotting setup for the test cases and some boiler plate for parsing and pre-processing the AST.

The phase 2 code is riddled with old unused stuff from Mope.

Future

This is literally the setup for this project and a proof of concept-y gimmick.

Limitations

While a lot of effort is done to keep the same semantics there are a few cases Preval decided to ignore.

These are edge cases, outdated code patterns, or things that often won't work in the wild anyways (like .name and minification).

Here is a list of such things. If your code depends on it, this is not the tool for you:

• The live secret binding of arguments is ignored
  • Reading arguments[0] to get param input values should work fine
  • Reading arguments.length should be reliable as well
• Function.name and Class.name will not work
  • These are often broken by minifiers and other tooling, anyways
• Some TDZ cases will not trigger TDZ
  • Some effort is applied to maintain TDZ errros if they were to happen but some are prevented
  • It's impossible to completely discover TDZ by static analysis and some transforms are just a lot simpler if TDZ is not something to concern too much about
• Generators may not be triggered
  • Although in many cases it should even if was an implicit side effect