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Executing Async Tasks Serially With Array#Reduce

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Executing Async Tasks Serially With Array#Reduce

Want to know how to execute async tasks serially without needing to use the async#reduce or async#series libraries. Let me show you how with the array#reduce method...

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Suppose that we’re assigned the task of writing a migration tool for a database with the following requirements:

  • The tool will read a list of SQL scripts and execute them serially one after another.
  • Each script will run once the previous script has completed.
  • If any script execution fails, no more scripts will execute.

This can be done by using a library such as async#reduce (or async#series)

async.reduce(files, Promise.resolve(), function(prevPromise, file, callback){
        return readFileAsync(file, {encoding: 'utf-8'});
        return execDbAsync(query);
}, function(err, result){


Without using an additional library, this executing async tasks serially problem can be solved by using the Array#reduce method:

files.reduce(function(prevPromise, curr, curIdx, arr){
    return prevPromise.then(function(){
        return readFileAsync(curr, {encoding: 'utf-8'});
        return execDbAsync(query);
}, Promise.resolve());

The above code will return a Promise which:

  • gets resolved when all chaining promises get resolved.
  • gets rejected when any of chaining promises get rejected.

Compared to using async#reduce, using the built-in Array#reduce method has some benefits:

  • No additional library needed
  • No callback
  • Less code as we don’t need to explicitly call “callback (data)” to notify a task completion

Execution order looks like bellow:

read (script 1)—> exec (script 1)—> read (script2)—> exec (script 2)—>…–> read (script n)—> exec (script n)

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array ,problem ,library ,migration ,tasks ,tool

Published at DZone with permission of Can Ho, DZone MVB. See the original article here.

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