For await syntax

Given that the for await syntax is a thing now, I think it would be very useful if the .each() method returned an async iterator instead of using a classical callback.

[PaulKiddle]

This is difficult because the row callback for each is synchronous, but for await would require an aync callback (even if you don’t actually use any async methods in the for body). To achieve this result you’d have to keep each row in a queue and wait for the row callback to finish before emitting the next result.

Something like this should do it:

function resultQueue(){
  let returnValue;
  const q = [];
  return {
    getNext(){
      if(q.length){
        return q.shift();
      }
      return new Promise(resolve => {
        returnValue = v => {
          returnValue = null;
          resolve(v);
        };
      });
    },

    setNext(v){
      if(returnValue) {
        returnValue(v);
      } else {
        q.push(v);
      }
    }
  }
}

async function* asyncEach(statement) {
  const { getNext, setNext } = resultQueue();
  db.each(statement, setNext).then(() => setNext(null));
  let row;
  // Assuming there are no falsy values passed to `setNext` until the final callback
  // i.e. null return value indicates no more records
  while(row = await getNext()) {
    yield row;
  }
}

(async function(){
  for await(const row of asyncEach('SELECT * FROM table;')) {
    await doSomethingWithRow(row);
  }
}());

[brerneto]

Why is there not a completed callback? An async queue is easy to do myself but I need to know when to end.

Edit: Sorry, just found the db property