How does Knex await execute a database query?

Question

I'm trying to understand how the await keyword is being used in KNEX. Take the following example:

knex.schema.createTable( ... );

this will return this which is an instance of SchemaBuilder. It will not execute the create table query in the database. However if I stick and await in front of it.

await knex.schema.createTable( ... );

This will now execute the create query in the database.

My understanding is that await is used to wait for a promise resolution but in this case it feels like something else is going on because not awaiting the function doesn't return a promise.

How does this work?

This was my understanding but if I don't `await` the function call it definitely doesn't create the table in the database. Compared with if I do use `await` it will execute the query and the table will appear in the database. — GeraldHost, Dec 16 '20 at 08:53
@T.J.Crowder The former expression just creates a SchemaBuilder, the latter expression calls the SchemaBuilder's `.then()` method. — Bergi, Dec 16 '20 at 13:19
@Bergi you are right! I didn't realise you could set up a then prototype that would be called when you use `await`. `await fn()` is just simply syntax sugar for `fn().then()`. Thank you! — GeraldHost, Dec 21 '20 at 14:00

xom9ikk · Accepted Answer · 2020-12-16T13:30:29.557

If you want to know how knex only makes requests when you write await in front of the construct, then here.

Under the hood, knex uses a pattern that returns an object with a then field.

const asyncFunction = (delay) => {
  return new Promise((resolve) => {
    setTimeout(() => {
      return resolve(delay);
    }, delay);
  })
}

const builder = (delay) => {
  return {
    then: async (resolve) => {
      const result = await asyncFunction(delay);
      return resolve(result);
    }
  }
}

const main = async () => {
  const array = [];
  for(let i=0; i<10; i++) {
    array.push(builder(i));
  }
  console.log('array', array);
  console.log('array[0]', array[0]);
  console.log('await array[0]', await array[0]);
  console.log('Promise.all for array', await Promise.all(array));
}

main();

The result of this execution will be the following output to the console

array [
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] },
  { then: [AsyncFunction: then] }
]
array[0] { then: [AsyncFunction: then] }
await array[0] 0
Promise.all for array [
  0, 1, 2, 3, 4,
  5, 6, 7, 8, 9
]

As you can see, the code inside the then function will not be called until the await keyword or other ways of waiting for Promise are used.

Don't forget the second parameter to `then`. Also I would recommend not to make `then` an `async function` to avoid weird handling of rejections. Just make `{ then(...args) { return asyncFunction().then(...args); } }`. — Bergi, Dec 16 '20 at 13:23

How does Knex await execute a database query?

1 Answers1