Description
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);
});