Remove unused transpiled Typescript enum

[mischnic]

Bug report or Feature request?

sub-optimal output

Version (complete output of terser -V or specific git commit)

5.12.1

Complete CLI command or minify() options used

terser -mc --module --toplevel index.js

terser input

// Typescript input to Babel
// enum Rec {
//   A = 123123,
//   B = 123124,
//   C = 123125,
//   D = 123126,
// }


var Rec;

(function (Rec) {
  Rec[Rec["A"] = 123123] = "A";
  Rec[Rec["B"] = 123124] = "B";
  Rec[Rec["C"] = 123125] = "C";
  Rec[Rec["D"] = 123126] = "D";
})(Rec || (Rec = {}));

console.log(123)

terser output or error

var o;!function(o){o[o.A=123123]="A",o[o.B=123124]="B",o[o.C=123125]="C",o[o.D=123126]="D"}(o||(o={})),console.log(123);

Expected result

Is there any change this could be removed (because Rec is unused)? This would have a rather high impact because it applies to all Typescript enums.

[mischnic]

One solution would of course be to fix TSC/Babel/swc to instead do something using /*@__PURE__*/

var Rec = /*@__PURE__*/ (function (Rec) {
  Rec[Rec["A"] = 123123] = "A";
  Rec[Rec["B"] = 123124] = "B";
  Rec[Rec["C"] = 123125] = "C";
  Rec[Rec["D"] = 123126] = "D";
  return Rec;
})({});

but I imagine this Rec || (Rec = {}) expression is there for a reason and changing all of the tools will be very difficult. And even after that, there's still transpiled code like this published on npm.

[evanw]

but I imagine this Rec || (Rec = {}) expression is there for a reason

That's for enum declaration merging. FWIW that's easy to solve for too: microsoft/TypeScript#27604 (comment). You just need to change ({}) into (Rec || {}) in your second comment and it should be exactly equivalent to TypeScript's current output (except easier to tree-shake). I've thought about this before because I actually changed esbuild's output to do this, and it has been working well.

[mischnic]

@evanw Great to hear that this is working without problems in esbuild! I might try to change this for swc and Babel.

Related Babel issue: babel/babel#9268
swc issue: swc-project/swc#4453

[fabiosantoscode]

This is a construct that I think is too tricky to address in general, but Terser already supports another TS pattern of using the identity function to avoid repeating as unknown as OtherType.

So this specific usage can be addressed specifically IMO.

[ehoogeveen-medweb]

Looks like this will be fixed on the babel side: babel/babel#15467

[fabiosantoscode]

Lovely!