async.js each get index in iterator

Question

I\'m using caolan\'s async.js library, specifically the .each method.

How do you get access to the index in the iterator?

async.each(ary, function(elemen         


        

Answer 1

The method async.each() will iterate the array in parallel, and it doesn't provide the element's index to the iterating callback.

So when you have:

function( done ){

  async.each(

    someArray,

    function( item, cb ){
      // ... processing

      cb( null );
    },

    function( err ){
      if( err ) return done( err );

      // ...
      done( null );
    }
  );
}

While you COULD use Array.indexOf() to find it:

function( done ){

  async.each(

    someArray,

    function( item, cb ){
      // ... processing

      var index = someArray.indexOf( item );

      cb( null );
    },

    function( err ){
      if( err ) return done( err );

      // ...
      done( null );
    }
  );
}

This requires an in-memory search in the array for EVERY iteration of the array. For large-ish arrays, this might slow everything down quite badly.

A better workaround could be by using async.eachSeries() instead, and keep track of the index yourself:

function( done ){

  var index = -1;
  async.eachSeries(

    someArray,

    function( item, cb ){

      // index is updated. Its first value will be `0` as expected
      index++;

      // ... processing

      cb( null );
    },

    function( err ){
      if( err ) return done( err );

      // ...
      done( null );
    }
  );
}

With eachSeries(), you are guaranteed that things will be done in the right order.

Another workaround, which is the async's maintainer's first choice, is to iterate with Object.keys:

function( done ){

  async.each(

    Object.keys( someArray ),

    function( key, cb ){

      // Get the value from the key          
      var item = someArray[ key ];

      // ... processing

      cb( null );
    },

    function( err ){
      if( err ) return done( err );

      // ...
      done( null );
    }
  );
}

I hope this helps.