Wait For Promises From A For Loop

The following code does not really do what I want. function doIt () { return new Promise (function (resolve, reject) { var promises = []; db.transaction(function(tx1){

Solution 1:

You can just move where the Promise.all() is located so that it's right AFTER the for loop has finished populating the array:

functiondoIt () {
  returnnewPromise (function (resolve, reject) {
    var promises = [];
    db.transaction(function(tx1){
      tx1.executeSql(function(tx2, rs) {
        for (var i = i; i < N; i++) {
          promises.push(db.transaction(function(tx3){
            ...
          }));
        }
        Promise.all(promises).then(resolve);
      });
    });

  });
}

FYI, mixing promises and callbacks can be confusing and makes consistent error handling particularly difficult. Does tx1.executeSql() already return a promise? If so, you can do something cleaner using only promises that are already created by your database functions like this:

functiondoIt() {
    return db.transaction.then(function(tx1) {
        return tx1.executeSql().then(function(tx2, rs) {
            var promises = [];
            for (var i = i; i < N; i++) {
                promises.push(db.transaction().then(function(tx3) {
                    ...
                }));
            }
            returnPromise.all(promises).then(resolve);
        });
    });
}

This returns promises from .then() handlers to auto-chain promises together.