support for `useDefineForClassFields`

[tmm1]

hoping for some pointers on how to start about fixing this:

❯ cat test.ts
class Point {
    x: number = 2;
    y: number;
}

❯ tsgo tsc -t es2020 /tmp/test.ts && cat /tmp/test.js
Files:               52
Types:               17794
class Point {
    x = 2;
    y;
}

❯ npx --package typescript tsc -t es2020 /tmp/test.ts && cat /tmp/test.js
class Point {
    constructor() {
        this.x = 2;
    }
}

[tmm1]

for reasons that are not clear to me, in the vscode codebase the generated syntax is es2020 style even though the tsconfig says es2022

❯ cd vscode
❯ grep es20 src/tsconfig.*
src/tsconfig.base.json:         "target": "es2022",

❯ echo 'class Point { x: number = 2; }' > src/point.ts
❯ npm run gulp compile-client
❯ cat out/point.js
"use strict";
class Point {
    constructor() {
        this.x = 2;
    }
}

[tmm1]

is the printer the right place for this?

diff --git i/internal/printer/printer.go w/internal/printer/printer.go
index 7b65f042a..1dee8bc9c 100644
--- i/internal/printer/printer.go
+++ w/internal/printer/printer.go
@@ -2843,6 +2843,7 @@ func (p *Printer) emitSpreadElement(node *ast.SpreadElement) {
 }
 
 func (p *Printer) emitClassExpression(node *ast.ClassExpression) {
+	target := p.currentSourceFile.LanguageVersion
 	state := p.enterNode(node.AsNode())
 	p.generateNameIfNeeded(node.Name())
 
@@ -2863,7 +2864,13 @@ func (p *Printer) emitClassExpression(node *ast.ClassExpression) {
 	p.writePunctuation("{")
 	p.pushNameGenerationScope(node.AsNode())
 	p.generateAllMemberNames(node.Members)
-	p.emitList((*Printer).emitClassElement, node.AsNode(), node.Members, LFClassMembers)
+	if target >= core.ScriptTargetES2022 {
+		p.emitList((*Printer).emitClassElement, node.AsNode(), node.Members, LFClassMembers)
+	} else {
+		// !!!
+		// add constructor declaration if missing
+		// move property decls into constructor body?
+	}
 	p.popNameGenerationScope(node.AsNode())
 	p.writePunctuation("}")
 

EDIT: would need to handle emitClassExpression too

[tmm1]

okay i see there is a transformation layer, so maybe this belongs in ESModuleTransformer or RuntimeSyntaxTransformer

[tmm1]

I think I misdiagnosed my issue and I don't actually need es2020 compat classes. Something else is causing generated JS not to load.

[tmm1]

for reasons that are not clear to me

"useDefineForClassFields": false

cc microsoft/vscode#186726

[jakebailey]

None of the downleveling is supported yet.

[tmm1]

makes sense. the other issue i ran into was decorators being emitted.

[jakebailey]

Right, that'd be the esnext transform which removes it.

[tmm1]

https://github.com/microsoft/TypeScript/blob/0693cc72e6ddb6c7468a14e05888efa2f43ac7b0/src/compiler/transformer.ts#L145-L153

    if (languageVersion < ScriptTarget.ESNext) {
        transformers.push(transformESNext);
    }

    if (!compilerOptions.experimentalDecorators && (languageVersion < ScriptTarget.ESNext || !useDefineForClassFields)) {
        transformers.push(transformESDecorators);
    }

    transformers.push(transformClassFields);

not clear what the equivalent of this is on the golang side. maybe func (e *emitter) getScriptTransformers()

[jakebailey]

getScriptTransformers is the closest, yes, but we still do not have any of the other transforms. I would not recommend using our emit yet; it's not ready outside limited syntax outside pure type erasure and some enum stuff.

[tmm1]

#819 added an esnext transform into a new internal package, so now there's a more obvious place to add classFields and decorator support in transformers.GetScriptTransformers

typescript-go/internal/transformers/transformer.go

Lines 92 to 93 in 3978f50

	// !!! transform native decorator syntax
	// !!! transform class field syntax