Variable declaration list comments from async functions are removed

[JoshuaKGoldberg]

TypeScript Version: 2.2.1

Code

async function example() {
    // result.value will be promise
    const promise = Promise.resolve("foo");
    await promise;

    // result.value will be "foo"
    return "foo";
}

Expected behavior:

Ignoring __awaiter and __generator, the output should include // result.value will be promise

Actual behavior:

function example() {
    return __awaiter(this, void 0, void 0, function () {
        var promise;
        return __generator(this, function (_a) {
            switch (_a.label) {
                case 0:
                    promise = Promise.resolve("foo");
                    return [4 /*yield*/, promise];
                case 1:
                    _a.sent();
                    // result.value will be "foo"
                    return [2 /*return*/, "foo"];
            }
        });
    });
}

[JoshuaKGoldberg]

On further exploration, it looks like this is only happening when a comment appears before a variable declaration list.

async function example() {
    // test abc
    let abc;

    // test def
    let def,
        // ghi
        ghi;


    // result.value will be promise
    const promise = Promise.resolve("foo");
    await promise;

    // result.value will be "foo"
    return "foo";
}

...becomes:

function example() {
    return __awaiter(this, void 0, void 0, function () {
        var abc, def, 
        // ghi
        ghi, promise;
        return __generator(this, function (_a) {
            switch (_a.label) {
                case 0:
                    promise = Promise.resolve("foo");
                    return [4 /*yield*/, promise];
                case 1:
                    _a.sent();
                    // result.value will be "foo"
                    return [2 /*return*/, "foo"];
            }
        });
    });
}

[yuit]

Talk with @rbuckton this turned out to be harder problem than it looks... Couple thing we need to figure out

• Where should the comment go when there are two separate variable statement comment?

async function example() {
    // test abc
    let abc;

    // test def
    let def,
    ....
}

function example() {
    return __awaiter(this, void 0, void 0, function () {
     --- what should this comment be -----
        var abc, def, 
    ....
}

• how we currently transform variable statement and hoist it declaration doesn't easily allow us to pass in information about the original variable statement (which we need to get the comment back)

[JoshuaKGoldberg]

Where should the comment go when there are two separate variable statement comment?

Seems reasonable to separate variable declarations by comments the way they are now.

async function example() {
    // test abc
    let abc;

    // test def, ghi
    let def, ghi;

    // test jkl
    let jkl,
        // test mno
        mno;
    ...
}

Output by default:

function example() {
    return __awaiter(this, void 0, void 0, function () {
        // test abc
        var abc;
        // test def, ghi
        var def, ghi;
        // test jkl
        var jkl,
        // test mno
        mno;
        ...
}

Output with --removeComments:

function example() {
    return __awaiter(this, void 0, void 0, function () {
        var abc, def, ghi, jkl, mno;
        ...
}

[rbuckton]

@JoshuaKGoldberg, I think the example here is not clear. Consider this:

async function example() {
  // comment1
  let a = 1;
  // comment2
  let b;
}

Becomes:

function example() {
  return __awaiter(this, void 0, void 0, function () {
    var a, b;
    return __generator(this, function (_a) {
      switch (_a.label) {
        case 0:
          a = 1;
          return [2 /*return*/];
      }
    });
  });
}

As a result of the transformation, the declaration of a has been separated from its initializer, and the declaration of b has been hoisted and is no longer associated with related code from the source position.

For a, we could choose either the hoisted declaration or the initializer as the location to place comments. For b, we can either choose the hoisted declaration or synthesize a NotEmittedStatement for that position so that we can associate its comment. Either choice could be considered reasonable by developers, so the situation is not quite so clear cut.

If we do opt to associate the comments with the declaration, it will require some non-trivial changes in the generators transform to emulate the comment emit in your recommendation.