v0.19.12

• The "preserve" JSX mode now preserves JSX text verbatim (#3605)

  The JSX specification deliberately doesn't specify how JSX text is supposed to be interpreted and there is no canonical way to interpret JSX text. Two most popular interpretations are Babel and TypeScript. Yes they are different (esbuild deliberately follows TypeScript by the way).

  Previously esbuild normalized text to the TypeScript interpretation when the "preserve" JSX mode is active. However, "preserve" should arguably reproduce the original JSX text verbatim so that whatever JSX transform runs after esbuild is free to interpret it however it wants. So with this release, esbuild will now pass JSX text through unmodified:

  // Original code
  let el =
    <a href={'/'} title='&apos;&quot;'> some text
      {foo}
        more text </a>
  
  // Old output (with --loader=jsx --jsx=preserve)
  let el = <a href="/" title={`'"`}>
    {" some text"}
    {foo}
    {"more text "}
  </a>;
  
  // New output (with --loader=jsx --jsx=preserve)
  let el = <a href={"/"} title='&apos;&quot;'> some text
      {foo}
        more text </a>;

• Allow JSX elements as JSX attribute values

  JSX has an obscure feature where you can use JSX elements in attribute position without surrounding them with {...}. It looks like this:

  let el = <div data-ab=<><a/><b/></>/>;

  I think I originally didn't implement it even though it's part of the JSX specification because it previously didn't work in TypeScript (and potentially also in Babel?). However, support for it was silently added in TypeScript 4.8 without me noticing and Babel has also since fixed their bugs regarding this feature. So I'm adding it to esbuild too now that I know it's widely supported.

  Keep in mind that there is some ongoing discussion about removing this feature from JSX. I agree that the syntax seems out of place (it does away with the elegance of "JSX is basically just XML with {...} escapes" for something arguably harder to read, which doesn't seem like a good trade-off), but it's in the specification and TypeScript and Babel both implement it so I'm going to have esbuild implement it too. However, I reserve the right to remove it from esbuild if it's ever removed from the specification in the future. So use it with caution.

• Fix a bug with TypeScript type parsing (#3574)

  This release fixes a bug with esbuild's TypeScript parser where a conditional type containing a union type that ends with an infer type that ends with a constraint could fail to parse. This was caused by the "don't parse a conditional type" flag not getting passed through the union type parser. Here's an example of valid TypeScript code that previously failed to parse correctly:

  type InferUnion<T> = T extends { a: infer U extends number } | infer U extends number ? U : never