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