section25.1.md

Section 25.1: Parallel : multi-tasking

async.parallel(tasks, afterTasksCallback) will execute a set of tasks in parallel and wait the end of all tasks (reported by the call of callback function).

When tasks are finished, async call the main callback with all errors and all results of tasks.

npm package name : npm i async

function shortTimeFunction(callback) {
  setTimeout(function() {
    callback(null, 'resultOfShortTime');
  }, 200);
}

function mediumTimeFunction(callback) {
  setTimeout(function() {
    callback(null, 'resultOfMediumTime');
  }, 500);
}

function longTimeFunction(callback) {
  setTimeout(function() {
    callback(null, 'resultOfLongTime');
  }, 1000);
}

async.parallel([
    shortTimeFunction,
    mediumTimeFunction,
    longTimeFunction
  ],
  function(err, results) {
    if (err) {
      return console.error(err);
    }
    console.log(results);
});

Result : ["resultOfShortTime", "resultOfMediumTime", "resultOfLongTime"] .

Call async.parallel() with an object

async.parallel({
    short: shortTimeFunction,
    medium: mediumTimeFunction,
    long: longTimeFunction
  },
  function(err, results) {
    if (err) {
      return console.error(err);
    }
    console.log(results);
});

Result : {short: "resultOfShortTime", medium: "resultOfMediumTime", long: "resultOfLongTime"}.

Resolving multiple values

async.parallel({
    short: function shortTimeFunction(callback) {
      setTimeout(function() {
        callback(null, 'resultOfShortTime1', 'resultOfShortTime2');
      }, 200);
    },

    medium: function mediumTimeFunction(callback) {
      setTimeout(function() {
        callback(null, 'resultOfMediumTime1', 'resultOfMeiumTime2');
      }, 500);
    }
  },

  function(err, results) {
    if (err) {
      return console.error(err);
    }
    console.log(results);
});

Result :

{
  short: ["resultOfShortTime1", "resultOfShortTime2"],
  medium: ["resultOfMediumTime1", "resultOfMediumTime2"]
}