Bug in async/await when doing a "return" statement inside of a for loop that awaits in an async function

[Zlatkovsky]

In current prototype of the Async/Await downlevel support, we found a bug causes an erroneous exception to occur in code that should otherwise work.

Repro:

The gist of the issue is that having a “return” statement inside of a for loop in a function that has an await in it causes an error.

Run this code an you’ll see that whenever the “num” is > .5 (and the “return true” statement gets evaluated), you get the following error that comes from within the __generator code: “TypeError: Unable to get property '0' of undefined or null reference”

    async function itemExists(): Q.Promise<boolean> {
        var numTries = 3;
        for (var i = 0; i < numTries; i++) {
            var num = await Q.fcall(function() { return Math.random() });
            console.log(num);
            if (num > .5) {
                return true;
            }
        }
        return false;
    }

    itemExists().then(function(val) {
        console.log("All values greater than .5: " + val)
    }).catch(function(e) {
        console.error("Error: " + e);
    });

Analysis:

The problem is caused by the “return” statement within the loop. If I had modified itemExists() to set a local variable, for example, and then break out of the loop via a “break” statement, the code would work:

    // With workaround (NOT short-circuiting the loop)
    async function itemExists(): Q.Promise<boolean> {
        var numTries = 3;
        var result = false;
        for (var i = 0; i < numTries; i++) {
            var num = await Q.fcall(function() { return Math.random() });
            console.log(num);
            if (num > .5) {
                result = true;
                break;
            }
        }
        return result;
   }

A quick analysis of the issue seems to be in the generated code. The exception is throw in here:

            try {
                var operation = body(state);
                opcode = operation[0], arg = operation[1];
            } catch (e) {
                opcode = 1 /*throw*/, arg = e;
            }

But the actual culprit seems to be in the fact that there is an inconsistency in the generated return and break statement (so far as I can tell). Namely, 3 looks like sometime a “break” and sometimes a “return”.

function itemExists() {
    return new Q.Promise(function (_resolve) {
        _resolve(__awaiter(__generator(function (_state) {
            switch (_state.label) {
                case 0:
                    numTries = 3;
                    i = 0;
                    _state.label = 1;
                case 1:
                    if (!(i < numTries))
                        return [3 /*break*/, 4];
                    return [4 /*yield*/, Q.fcall(function () {
                        return Math.random();
                    })];
                case 2:
                    num = _state.sent;
                    console.log(num);
                    if (num > .5) {
                       return [3 /*return*/, true];
                    }
                    _state.label = 3;
                case 3:
                    i++;
                    return [3 /*break*/, 1];
                case 4:
                    return [2 /*return*/, false];
            }
        })));
    });
    var numTries, i, num;
}
itemExists().then(function (val) {
    console.log("All values greater than .5: " + val);
}).catch(function (e) {
    console.error("Error: " + e);
});

[d180cf]

This state machine looks like a downlevel emit of generators. Does it mean that tsc@1.7 is going to support yield when targeting es5?

[mhegazy]

@d180cf we are working on it :)

[rbuckton]

This should no longer be an issue as of 4685646, which is the new version of these downlevel transformations for async functions we are targeting for our 2.1 release.