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 :)