What is the 'this' Binding in Async 'seq' and 'compose' Functions?

TLDR: global

Async is a powerful utility module for working with asynchronous programming style in JavaScript. In the documentation, there are a couple places talking about this:

Each function is executed with the this binding of the composed function. - async

This affects these two functions:

  • seq(fn1, fn2...)
  • compose(fn1, fn2...)

When printing the this context from a composed function, this is what we have:

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[ 'global',
  'process',
  'GLOBAL',
  'root',
  'Buffer',
  'setTimeout',
  'setInterval',
  'clearTimeout',
  'clearInterval',
  'setImmediate',
  'clearImmediate',
  'console' ]

This looks very much like the global namespace object.

Let’s verify it:

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// Async: This Binding
// ===================
//
// Find out the `this` binding in Async composed functions.
'use strict';
// Dependencies
// ------------
var async = require('async');
// Functions
// ---------
function fn1(done) {
  console.log('fn1: this === global ?', this === global);
  setTimeout(function () {
    done();
  }, 10);
}
function fn2(done) {
  console.log('fn2: this === global ?', this === global);
  setTimeout(function () {
    done();
  }, 10);
}
// Main
// ----
var fns = async.seq(fn1, fn2);
fns(function (err) {
  if (err) {
    throw err;
  }
});

The result:

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fn1: this === global ? true
fn2: this === global ? true

This is due to the behavior of this in Node.js:

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console.log(this === global);  // false
console.log(this === exports); // true
(function(){ console.log(this === global); }());  // true      
(function(){ console.log(this === exports); }()); // false