gistfile1.js

// Here is a simple object with an (unnecessarily roundabout) squaring method
var AsyncSquaringLibrary = {
  squareExponent: 2,
  square: function(number, callback){ 
    var result = Math.pow(number, this.squareExponent);
    setTimeout(function(){
      callback(null, result);
    }, 200);
  }
};

async.map([1, 2, 3], AsyncSquaringLibrary.square, function(err, result){
  // result is [NaN, NaN, NaN]
  // This fails because the `this.squareExponent` expression in the square
  // function is not evaluated in the context of AsyncSquaringLibrary, and is
  // therefore undefined.
});

async.map([1, 2, 3], AsyncSquaringLibrary.square.bind(AsyncSquaringLibrary), function(err, result){
  // result is [1, 4, 9]
  // With the help of bind we can attach a context to the iterator before
  // passing it to async. Now the square function will be executed in its 
  // 'home' AsyncSquaringLibrary context and the value of `this.squareExponent`
  // will be as expected.
});