Static class fields can’t be tree-shaken away on subsequent compilation

Hey Evan,

Thank you for creating and maintaining esbuild!

We @ Framer stumbled upon an issue around static class fields. When you use them, and you’re compiling for ES2021 and below, esbuild produces code that’s not tree-shakeable on a subsequent compilation. This is relevant e.g. when you’re a library author and are distributing the library as a bundle.

Steps to reproduce

• Create my-library.js with the following code:

  export class Navigation {
    state = defaultState()
  
    static defaultProps = {
      enabled: true,
    }
  
    static contextType = NavigationCallbackContext
  }
  

• Bundle the library with { target: "es2019" } into my-library-compiled.js. Observe that the library is compiled down to:

  var __defProp = Object.defineProperty;
  var __defNormalProp = (obj, key, value) => key in obj ? __defProp(obj, key, { enumerable: true, configurable: true, writable: true, value }) : obj[key] = value;
  var __publicField = (obj, key, value) => __defNormalProp(obj, typeof key !== "symbol" ? key + "" : key, value);
  export class Navigation {
    constructor() {
      __publicField(this, "state", defaultState());
    }
  }
  __publicField(Navigation, "defaultProps", {
    enabled: true
  });
  __publicField(Navigation, "contextType", NavigationCallbackContext);
  

• Now, import the compiled library from another file (say, my-app.js):

  import {} from "./my-library-compiled.js"
  

• Bundle that file (using esbuild, webpack, etc – doesn’t matter). Observe that the app bundle includes Navigation, even though it’s not used.

Actual result

Any classes that use static fields in the library can’t be tree-shaken away due to top-level __publicField setters.

Expected result

esbuild compiles classes with static fields down to something like this:

export class Navigation {
  state = defaultState()

  static defaultProps = {
    enabled: true,
  }

  static contextType = NavigationCallbackContext
}

↓

export var Navigation = /* @__PURE__ */ (() => {
  class Navigation {
    constructor() {
      __publicField(this, "state", defaultState());
    }
  }
  
  __publicField(Navigation, "defaultProps", {
    enabled: true
  });
  
  __publicField(Navigation, "contextType", NavigationCallbackContext);
  
  return Navigation;
})()

which allows Navigation to be tree-shaken away.

Start with the linked esbuild reproduction and compare the two compilation stages described in the issue. Trace static class-field lowering and subsequent tree shaking, then verify that an unused class with lowered static fields is removed without changing the output for a used class.