evanw/esbuild v0.18.5

on GitHub

Implement auto accessors (#3009)

This release implements the new auto-accessor syntax from the upcoming JavaScript decorators proposal. The auto-accessor syntax looks like this:

class Foo {
  accessor foo;
  static accessor bar;
}
new Foo().foo = Foo.bar;

This syntax is not yet a part of JavaScript but it was added to TypeScript in version 4.9. More information about this feature can be found in microsoft/TypeScript#49705. Auto-accessors will be transformed if the target is set to something other than esnext:

// Output (with --target=esnext)
class Foo {
  accessor foo;
  static accessor bar;
}
new Foo().foo = Foo.bar;

// Output (with --target=es2022)
class Foo {
  #foo;
  get foo() {
    return this.#foo;
  }
  set foo(_) {
    this.#foo = _;
  }
  static #bar;
  static get bar() {
    return this.#bar;
  }
  static set bar(_) {
    this.#bar = _;
  }
}
new Foo().foo = Foo.bar;

// Output (with --target=es2021)
var _foo, _bar;
class Foo {
  constructor() {
    __privateAdd(this, _foo, void 0);
  }
  get foo() {
    return __privateGet(this, _foo);
  }
  set foo(_) {
    __privateSet(this, _foo, _);
  }
  static get bar() {
    return __privateGet(this, _bar);
  }
  static set bar(_) {
    __privateSet(this, _bar, _);
  }
}
_foo = new WeakMap();
_bar = new WeakMap();
__privateAdd(Foo, _bar, void 0);
new Foo().foo = Foo.bar;

You can also now use auto-accessors with esbuild's TypeScript experimental decorator transformation, which should behave the same as decorating the underlying getter/setter pair.

Please keep in mind that this syntax is not yet part of JavaScript. This release enables auto-accessors in .js files with the expectation that it will be a part of JavaScript soon. However, esbuild may change or remove this feature in the future if JavaScript ends up changing or removing this feature. Use this feature with caution for now.

Pass through JavaScript decorators (#104)

In this release, esbuild now parses decorators from the upcoming JavaScript decorators proposal and passes them through to the output unmodified (as long as the language target is set to esnext). Transforming JavaScript decorators to environments that don't support them has not been implemented yet. The only decorator transform that esbuild currently implements is still the TypeScript experimental decorator transform, which only works in .ts files and which requires "experimentalDecorators": true in your tsconfig.json file.

Static fields with assign semantics now use static blocks if possible

Setting useDefineForClassFields to false in TypeScript requires rewriting class fields to assignment statements. Previously this was done by removing the field from the class body and adding an assignment statement after the class declaration. However, this also caused any private fields to also be lowered by necessity (in case a field initializer uses a private symbol, either directly or indirectly). This release changes this transform to use an inline static block if it's supported, which avoids needing to lower private fields in this scenario:

// Original code
class Test {
  static #foo = 123
  static bar = this.#foo
}

// Old output (with useDefineForClassFields=false)
var _foo;
const _Test = class _Test {
};
_foo = new WeakMap();
__privateAdd(_Test, _foo, 123);
_Test.bar = __privateGet(_Test, _foo);
let Test = _Test;

// New output (with useDefineForClassFields=false)
class Test {
  static #foo = 123;
  static {
    this.bar = this.#foo;
  }
}

Fix TypeScript experimental decorators combined with --mangle-props (#3177)

Previously using TypeScript experimental decorators combined with the --mangle-props setting could result in a crash, as the experimental decorator transform was not expecting a mangled property as a class member. This release fixes the crash so you can now combine both of these features together safely.