Use --preserveConstEnums or switch to non-const enums

[DanielRosenwasser]

The TypeScript team largely regards const enums as a design mistake because it causes so many downstream issues with libraries.

One issue I've personally run into with Fluid though relates to the fact that they don't work with the single-file at a time compilation model that lots of tools use including

• TypeScript's transpileModule (which is surfaced by ts-loader and awesome-typescript-loader)
• Babel
• esbuild
• swc

As an example, if you try to write a line of code like

import { IMergeTreeOp, MergeTreeDeltaType } from "@fluidframework/mergetree";

function mergeymerge(op: IMergeTreeOp) {
  if (op.type === MergeTreeDeltaType.INSERT) {
    // ...
  }
}

The code will fail to run correctly under several of these modes, and you will get an error under --isolatedModules.

One way to alleviate issues with const enums is to at least compile the TypeScript code with --preserveConstEnums, so that these exports exist at runtime or bundle-time. These can increase your bundle size if you're not using TypeScript, but it will make running the code more accessible.

[curtisman]

@DanielRosenwasser From reading the docs, it seems like we should turn on --isolatedModules to catch similar type of errors to support single file transpilers as well?

[DanielRosenwasser]

Yeah, that will help avoid this sort of code in the future, and I turned it on in my own project.

[curtisman]

@DanielRosenwasser so I turned on preserveConstEnums in merge-tree, and turned on isolatedModules in examples\data-objects\math, which imports MergeTreeDeltaType. It still gives me an error for the examples\data-objects\math

@fluid-example/math: src/mathComponent.ts:654:87 - error TS2748: Cannot access ambient const enums when the '--isolatedModules' flag is provided.
@fluid-example/math:
@fluid-example/math: 654                     instance.remoteEdit(pos - startPos, len, event.deltaOperation === MergeTree.MergeTreeDeltaType.INSERT);

It looks like preserveConstEnums only force code to be generated for the enum, but not changing the type, so the consumer of it with isolatedModules flag would still complain? Is that by design that project that uses isolatedModules flags can not consume any project that has const enum no matter what flags is used to build that project?

[DanielRosenwasser]

Hm, well under isolatedModules it might be unclear whether a const enum was compiled with preserveConstEnums. We could special-case it for non-declaration files, but this still seems like a pain-point I'll discuss with the TS team.