Node.js wait until each iteration in for loop

Question

I have a for loop

for (let i = 0; i < options.length; i++) {
        console.log("Entered the to for " + i);
        let employee = await this.prolesChecker.getEmployeesFromEmail(options[i]);
        let isOnVacation = await this.prolesChecker.isOnVacation(employee, moment());
    }

The two functions "getEmployeesFromEmail and isOnVacation" are connecting to a database and they need some time until result is returned. I want the for loop to wait until the result is returned and then go in the next iteration.

As an example the console.log always prints

Entered the to for 0

It never goes to i = 1

Here is the function

 public async deleteEmailsTo(options: any) {
    console.log(options.length);
    for (let i = 0; i < options.length; i++) {
        console.log("Entered the to for " + i);
        let employee = await this.prolesChecker.getEmployeesFromEmail(options[i]);
        let isOnVacation = await this.prolesChecker.isOnVacation(employee, moment());
        if ((!employee.EmailReminders && !isOnVacation) || (!employee.EmailReminders && !employee.EmailRemindersForHoliday && isOnVacation)) {
            let index = options.indexOf(options[i], 0);
            if (index > -1) {
                options.splice(index, 1);
                console.log("Removed " + employee.Name + " " + employee.LastName + " from the 'to' list");
            }
        }
    }
}

Any suggestions please?

If the for loop in a function? If so, the function has to be async — Julian Zucker
– Julian Zucker, Commented Aug 29, 2017 at 12:41
The loop already does wait until the returned promises are fulfilled with the results. If it waits forever, that suggests the returned promises don't resolve properly. Show us the relevant code of getEmployeesFromEmail and isOnVacation — Bergi
– Bergi, Commented Aug 29, 2017 at 12:43
@JulianZucker It is, otherwise it would immediately throw a syntax error instead of advancing until the first console.log — Bergi
– Bergi, Commented Aug 29, 2017 at 12:44
@Subburaj No, he shouldn't. Promises with async/await syntax are far superior to the async.js module. — Bergi
– Bergi, Commented Aug 29, 2017 at 12:45
I found the error was in other function "getEmployeesFromEmail" that was doing "splice" on the options array. — DavidS
– DavidS, Commented Aug 29, 2017 at 13:03

Bergi · Accepted Answer · 2017-08-29 13:02:50Z

1

Your problem is actually not about the async/await syntax, which works fine. It's about the use of splice during the iteration! That changes the .length of your array, and the next iteration won't happen simply because the loop condition didn't apply any more.

If you absolutely have to mutate the passed array, decrement the index counter to account for the change in length:

for (let i = 0; i < options.length; i++) {
    …
    if (…) {
        let index = i; // no point in using `options.indexOf(options[i], 0);`
        // if (index > -1) { always true
        options.splice(index--, 1);
//                     ^^^^^^^
    }
}

But it's probably much easier to simply create a new array:

let result = [];
for (let i = 0; i < options.length; i++) {
    …
    if (!…) {
        result.push(options[i]);
    }
}
return result;

answered Aug 29, 2017 at 13:02

Bergi

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