Mocha TDD like DSL in less than 100 lines

TL;DR A minimal synchronous mocha like DSL, in less than 100 lines of JavaScript. Code at

Our of curiosity, I tried to implement a Mocha like DSL. Seems like, the core of the DSL, could be written quite succinctly. First we will describe the core DSL.

Minimal Mocha DSL details

The 4 functions below, seem to be the relevant ones.

describe(desc, fn)
it(desc, fn)

setup, teardown and it are used inside a describe block.

The important part is, describe could be nested within describe, to arbitrary depth. ex:

describe('top level describe', function (){
    // ...
    describe('nested describe', function () {
        // ...
        describe('nested nested describe', function () {
            // ...
        // ...
    // ...

So each describe could have setup, teardown, it and well describe calls.

At any level, all functions registered in setup should be run first. Then all the tests registered with it, and then all the nested describe have to be executed. After all that, function registered with teardown should be executed. Ex:

So, how do we solve it.

Lets first think just one level (ex: ignore nested describes). We would have to collect all the DSL commands at that level, and then execute them in a specific order. Note that we can’t just execute the commands in the order we come across. Example, a setup might come after tests (it). But all the setup in a level have to be run before the tests. The way I achieve this is to define all the DSL commands as functions which save the command, and its arguments, to be executed later. ex: setup(fn) would save a pair ["setup", fn]. ex:

// 'key' being the dsl command, and 'val' arguments
function spush(key, val){

We will come to the stack part later. Now, once all the commands in a describe level are collected, they are to be executed one by one. Execution of setup and teardown commands is simple. Just execute the function registered with them, in the context (there is a single context, on which all setup and teardown work). Test (functions registered with it) are executed by running the registered function, and showing success or failure based on output. For now, it just checks for the exception thrown by the function. Ex:

function reportTests(fn, desc) {
    desc = test_title(stack, desc);
    try {;
    } catch(e) {
        failure(desc, e.message);

Describe is the special case. Executing it, essentially means doing the above, at the nested level (ex: the function registered with the describe). Note that teardown should run at the end. So, some information about the current level has to be saved, till all the nested levels are done. This is the reason to have stack, to keep the commands. describe is kind of the main function of the DSL.

function exec_describe(title, tfn) {
    // collect 'describe', 'setup', 'teardown' and 'it' for the current level;
    exec_top();  // execute them

Well, thats mostly it. There are some small details. Like directly executing the describe the first time we come across it (the process has to start somewhere). Or printing the titles etc.

For more details look at the code. Its nice to find out that an expressive DSL could be implemented (to at least experiment) in less than 100 lines of code.

PS: ActiveSphere is hiring. If you would like to join, try your hands at making this code handle asynchronous tests. You can find the problem description here. Visit our careers page. Send us a mail at

Sometimes, hard problems need a small experienced team, and a new perspective.

Know problems where we could be of help? Let’s Talk