Break a for loop from inside a promise in Sequelize

Question

I'm trying to break a loop inside a sequelize promise when it finds its first result and returns the first response, but it always do the entire loop. If I assign a value to a variable inside the promise, the value disappears outside. I've trying very hard for days and I will really appreciate if anyone can help me. This is my code:

for (let i = (parseInt(req.query.units) - 1); i >= 0; i--) {
  console.log (i);

  db.order_planned.findAll({
    where: {
      barcode: parseInt(req.query.barcode) + i,
      stepdesc: req.query.stepdesc
    },
    limit: 1,
  }).then((data) => {
    if (data[0].length > 0) {
      data[0].barcode = (parseInt(data[0].barcode) - i);
      console.log(data[0]);
      res.status(200).json(data[0]);
      break; // illegal break
    }
  }).catch((error) => {
    let err_msg = new ErrorMsg(error);
    res.status(err_msg.status).json(err_msg);
  });
}

Does this answer your question? [JavaScript ES6 promise for loop](https://stackoverflow.com/questions/40328932/javascript-es6-promise-for-loop) — rveerd, Jan 26 '21 at 15:08

score 2 · Accepted Answer · answered Jan 26 '21 at 16:02

You want to loop over n promises and return the first promise that resolves. Promise.any() does just that.

Using a simplified version of your code:

const promises = [];
for (let i = (parseInt(req.query.units) - 1); i >= 0; i--) {
  promises.push(
      db.order_planned.findAll({ /*..*/ }).then((data) => {          
          /*..*/
          return data[0]; // Found your data
      });
  );
}

// As soon as the first promise is returned, log it.
Promise.any(promises).then(firstResult => console.log(firstResult));

Because of the way promises work, the loop will always execute in it's entirety before any promises are returned.

Alternatively, you can use async/await to get what you desire:

for (let i = (parseInt(req.query.units) - 1); i >= 0; i--) {
    const data = await db.order_planned.findAll({ /*..*/ }); // Use await here
    if (data[0].length > 0) {
      /*..*/
      break; // Can now do a break here
    }
}

However, this will have worse performance because the promises are not executed in parallel.

Break a for loop from inside a promise in Sequelize

1 Answers1