The Done callback

What is the done callback and why is it important?

Problem

Concurrent Tasks is a JavaScript module, which runs multiple tasks concurrently until all the tasks are complete. It needs a way to figure out when a particular task has been completed.

Solution

Gulp solves this problem by either accepting a return of a Gulp task, or by calling a function done. Similarly, to solve the exact same problem, each task passed to the TaskRunner has access to a special function called done (ingenuity max).

Purpose

The purpose of this function is simple: Tell the instance when a particular task is complete!

Internally, the done function does a fair amount of work:

• Makes a free slot available for the internal runner
• Updates completion counts and calls the internal runner
• Updates the time elapsed from start, until the function calling done’s completion
• Calls the internal runner to pick up the next task in the queue

Examples

It’s a very simplistic approach, which can be used across anything. Be it functions, Promises, timeouts etc.

Functions

import { TaskRunner } from "concurrent-tasks";

const runner = new TaskRunner();

function randomNumbers(done) {
  const numbers = [];
  let count = 1000;
  while (count) {
    numbers.push(Math.round(Math.random() * 100));
    count--;
  }
  done();
}

runner.add(randomNumbers);

Promises

import { TaskRunner } from "concurrent-tasks";

const runner = new TaskRunner();

function getGoogle(done) {
  fetch("http://www.google.com")
    .then((data) => {
      console.log("data", data);
      done();
    })
    .catch((error) => {
      console.log("error", error);
      done();
      runner.add(getGoogle); // retry fetching data
    });
}

runner.add(getGoogle);

Timeouts

import { TaskRunner } from "concurrent-tasks";

const runner = new TaskRunner();

function afterTimeout(done) {
  const timeout = Math.floor(Math.random() * 1000);
  setTimeout(() => {
    console.log(`This resolved after ${timeout}ms`);
    done();
  }, timeout);
}

runner.add(afterTimeout);