Experimental Tree Shaker

[WIP] This is an experimental tree shaker for JS based on Oxc.

If this project goes well, I personally hope it can become part of the Oxc or the Rolldown project.

• Try it online: https://kermanx.github.io/tree-shaker/
• Test262 Result: Goto commits and view the latest comment

Features

• Simulate the runtime behavior of the code, instead of applying rules.
• Single AST pass - Analyzer as much information as possible.
• As accurate as possible. test262 is used for testing.
• May not be the fastest. (But I will try my best)

Demo

Tree shake the following code (this already works!):

export function f() {
  function g(a) {
    if (a) console.log("effect");
    else return "str";
  }
  let { ["x"]: y = 1 } = { x: g("") ? undefined : g(1) };
  return y;
}

To:

export function f() {
  return 1;
}

Although the code above is very simple to analyze, but achieving this effect correctly for every codebase requires a lot of work - JS is too dynamic.

Comparison

• Rollup: Rollup is great but somehow I couldn't understand its codebase. I hope this project can be more readable and maintainable. Currently, for the same code, this project can produce code roughly 2% smaller than Rollup (both after minification via uglify-js) for a Vue starter app.
• Closure Compiler: Closure Compiler can be considered as a tree shaker + minifier, while this project is only a tree shaker (for the minifier, we have oxc_minifier). Theoretically, we can shake more than Closure Compiler, but we cannot compare them directly because we don't have a equivalent minifier. Also, it's written in Java, which is hard to be integrated into the JS ecosystem.
• swc: swc can also be considered as a tree shaker + minifier. TBH, currently swc is much faster than this project and more complete. But it's rule-based, which is a different approach from this project. It's also not compatible with the Oxc project, thus a new tree shaker is needed.

Todo

• Performance!
• JS Builtins metadata
• Pure notation support
• Test against fixtures from other tree shakers like Rollup
• Rollup-like try-scope optimization/de-optimization
• Reuse code with oxc_minifier for JS computation logics
• Type narrowing
• Multiple-module support

Basic Approach

1. Parse the code via oxc_parser.
2. Build the semantic information via oxc_semantic.
3. Tree shake the code.
   • Emulate the runtime behavior of the code. (Control flow, Side effects, ...)
   • Analyze the possible runtime values of the variables.
   • Remove the dead code.
4. Minify the code via oxc_minifier. (Optional)

Concepts

• Entity: Represents the analyzed information of a JS value.
• Consumable: Entity or AST Nodes or some other things that the runtime value of Entity depends on.
• Scopes:
  • Call Scope: Function call scope.
  • Cf Scope: Control flow scope.
  • Variable Scope: Variable scope.
  • Try Scope: Try statement or function.