Add option to don't drop comments

[timocov]

Related to #218 and #220.

Tools like uglifyjs or terser may rely on some comments like /*@__PURE__*/ (see side_effects on https://github.com/terser/terser/blob/504b9677d2d6086025ead5afb713e151877944b2/README.md#compress-options).

In some projects you can't use TypeScript in transpileOnly mode (or don't want because you have already compiled code and just want to process JS files itself). The TypeScript leaves a comment like /** @class */ right before class' IIFE, which could be replaced (somehow) with /*@__PURE__*/ so terser/uglifyjs can understand it and do its magic with them (see mishoo/UglifyJS#2279).

It seems that esbuild drop all comments (licences as well) which does all comments-dependent processing unreal (dropping licences is the issue as well).

[evanw]

The esbuild parser was designed for speed. It's not designed to be a code formatter. It will likely be extremely difficult to modify esbuild to ensure that all comments are preserved and in their original locations, especially as code can be moved around due to syntax transformations. I don't think esbuild is the right tool if you need to do this, and I am not planning on adding general comment preservation to esbuild. You will want to use other tools to do this instead.

I can see adding a special case for statement-level comments only. I believe I should be able to guarantee that those are preserved without rewriting the parser. That should be sufficient to cover license comments I think.

[timocov]

I can see adding a special case for statement-level comments only.

Is that comments "attached" to a nodes? I mean a comments right before a node something like it's in TypeScript AST). If so, I think it might cover cases with "class" comment as well.

[evanw]

Is that comments "attached" to a nodes?

Attaching comments to nodes makes it easy for them to be accidentally lost. Instead I'm planning on creating a new kind of statement AST node for statement-level comments.

[evanw]

Relevant to this issue: I'm currently adding limited support for __PURE__ comments in esbuild (interpreting them, not passing them through). It's being done as part of some ongoing work for tree shaking. See #86 (comment) for context.

[evanw]

I just published a release with support for __PURE__ comments. I also added pass-through printing of them when minification is disabled.

Edit: Note that this is only for __PURE__ comments that esbuild recognizes, which are only those that become attached to CallExpressions and NewExpressions. Like I said above esbuild wasn't designed for general-purpose comment preservation.

[evanw]

I closed this because both __PURE__ comments and license comments have been implemented. I believe that should be sufficient for your use case.