Mocha Testing Tip for async code

Do you ever have a test fail just sometimes? If you are an experienced programmer, just the thought of it would have given you that drop of cold sweat, trickling down your backbone. Here is a recent example - I was altering a save function, that uses promises; changing it to do an atomic save. Simplified, my save() function started off something like this:

return this.saveHelper(fname)

saveHelper() returns a promise that resolves to true on success.

I then changed it to:

return this.saveHelper(tmpFname)
  .then(ret => {fs.rename(tmpFname, fname); return ret })

All my tests pass. Then 10 seconds later, they didn’t. Cue: cold sweat.

Doh! rename() (in the excellent fs-extra module) is async. I was returning before the promise had resolved. The fix was this (i.e. pass through the real return value we are interested in):

return this.saveHelper(tmpFname)
  .then(ret => fs.rename(tmpFname, fname).then(() => ret))

All my tests pass… but they did that before. How can I be sure I’ve fixed it?

What I do is use a bash infinite loop, to just run that particular test over and over:

 while ./node_modules/.bin/mocha --harmony test/mytest.js;do :; done

In my case the test takes less than half a second, so I let it run for 30 seconds, before using ctrl-c to break out. The nice thing about using while, is it will only stop if the test fails; otherwise it will just run forever. So you can start it running, go make a cup of tea, and if it is still running when you get back, chances are you fixed the bug.

(Depending on what you are doing, you may not need the —harmony flag, and you may need other flags to mocha.)

Naturally, you can also do this with your full test suite:

while yarn test;do :; done