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helpers for mocha, when using bluebird promises


These are a few small helpers for mocha, that simplify your test boilerplate if you use bluebird.

Each function is intended to be a mixin for a unit test, so for example, if you have a test like this:

it("adds numbers", function () {
  (2 + 2).should.eql(4);

you can add each helper as a wrapper around the test function, like this:

it("adds numbers", function () {
  Promise.resolve(2 + 2).then(function (sum) {

The helpers


Wrap a test function that returns a future:

  • future(function)
    • function: () => Promise code to execute as a test

Mocha's done hooks are attached to the future so that the test doesn't complete until the future is resolved.

it("waits 100 ms", future(function () {
  return Q.delay(100);


Wrap a test function so that it runs inside a temporary folder:

  • withTempFolder(function)
    • function: (folder) => Promise code to execute while the folder exists

The folder's name is passed as the first parameter to the function, and it's expected to return a Promise. When the Promise is resolved, the temporary folder is deleted (along with any contents).

This function requires future also, since the cleanup is attached to the result future.

it("creates a file", future(withTempFolder(function (folder) {
  fs.writeFileSync(folder + "/new-file", "hello!");


Execute a program as a future:

  • exec(command, options)
    • command: passed to child_process.exec
    • options: passed to child_process.exec

The parameters are passed to child_process.exec as-is and a future is returned. If the exec is successful, the future is resolved with an object with these fields:

  • process: the process object
  • stdout: the stdout buffer
  • stderr: the stderr buffer

If the exec fails, the future is rejected, and the error object will have those three fields added to it.

it("runs echo", future(function () {
  return exec("echo hello").then(function (result) {


Try running assertions a few times, until they pass (or run out of time):

  • eventually(options, function)
    • options:
      • timeout: milliseconds to wait before giving up (default: 1000)
      • frequency: milliseconds to wait between attempts (default: 50)
    • function: code to execute on each attempt

To assert that a background process will have some effect in a non-deterministic (but relatively short) time, you can group your assertions in an eventually block. If an exception is thrown the first time -- for example, by a failing assertion -- it will delay for a small period of time, then try again, repeating until the code executes without exception or the time runs out.