Creating Your Own Javascript Testing Framework

Testing is essential to development. I've obviously used testing libraries such as Mocha, Chai, Ava, Tape, etc. But how would you go about writing one yourself? How would you build a test framework using that same framework to test itself? I got to thinking, just what would a test runner or 'test harness' consist of? I thought the basics would be:

  • It should be able to look for test file/s and execute tests within
  • It should be able to make assertions within an executed test
  • It should be able to report successes and failures of these assertions

So we would have a command line tool. We would run and record tests and their assertions and record results into a report that is delivered as the conclusion of the tests.


The term harness, or 'test framework', is the thing that keeps track of the tests and sends the result to the reporter.


Assertions are statements of facts or beliefs. So they are binary; that an assertion is either correct or wrong. There are many styles of assertions, some prefer a string like method such as expect(value) or simply assert(condition).

General assertions I included:

  • 'assert', 'ok', 'isOk': truthy
  • 'assertNot', 'not', 'notOk': falsy
  • 'throws', 'throw': expect an exception to be thrown
  • 'doesNotThrow', 'notThrow': will not throw an exception
  • 'equal', 'equals', 'isEqual', 'is', 'strictEqual', 'strictEquals', 'strictIs', 'isStrict', 'isStrictly': equality condition
  • 'deepEqual', 'deepEquals', 'isDeepEqual', 'isDeep', 'strictDeepEqual', 'strictDeepEquals': object/array equality condition
  • 'notEqual', 'inequal', 'notEqual', 'notEquals', 'notStrictEqual', 'notStrictEquals', 'isNotEqual', 'isNot', 'doesNotEqual', 'isInequal': object/array not equal
  • 'notNull', 'isNotNull': specific type assertion

Helper assertions:

  • 'isAbove', 'above', 'greaterThan', 'over': numeric value above
  • 'isBelow', 'below', 'lessThan': numeric value below
  • 'isAtLeast', 'atLeast', 'greaterThanOrEqual': numeric value less than or equal
  • 'isFalse': falsy value
  • 'isTrue': truthy value

Included on the assert function is an 'expect' function, so you can destructure and use like this:

test('using expect' ({ expect }) => {
  • 'equal': equality test
  • 'ok': truthy test


I decided to use TAP (test anything protocol) for the default reporter.


This was a hugely beneficial exercise. Figuring out where to start was a challenge. I started with the assert statement as that is at the core of a test. All development was test driven too, so like Uncle Bob teaches - fail, pass, refactor.

At times very challenging, but rewarding. There are lots of things I would do differently if I did it again though. Check out the repo here.