Testing asynchronous code

The asynchronous nature of the Node.js platform means accounting for asynchronous behavior in tests. Fortunately, the Node.js unit test frameworks help in dealing with this, but it's worth spending a few moments considering the underlying problem.

Consider a code snippet like this, which you could save in a file named deleteFile.js:

const fs = require('fs');
exports.deleteFile = function(fname, callback) {
 fs.stat(fname, (err, stats) => {
 if (err)
  callback(new Error(`the file ${fname} does not exist`));
 else {
  fs.unlink(fname, err2 => {
   if (err)
   callback(new Error(`could not delete ${fname}`));
   else callback();
  });
 }
 });
};

The nature of asynchronous code is such that its execution order is nonlinear, meaning that there is a complex relationship between time and the lines of code. Since this is the real world, and not science fiction, we don't have a time machine (blue box or not) to help us traverse the web of time, and therefore, we must...