1.0.2 • Public • Published


    Test is about data, not more code. While writing a piece of code, we want to see it alive and feed it with data. jfest does exactly that.

    jfest is a lightweight command-line tool that automates your unit tests for JavaScript. It watches your .js files, tracks your function calls and safely executes them to show you the results. Writing or modifying a function? Use $test to see its returned values. If you have saved a previous execution, you'll even get a regression check. Happy with the results? Rename $test to $save and your test cases are now persistent. Finally, remove all $test and $save from your files. Your tests are safe, your code is clean.

    jfest is non-invasive. Zero test files, zero configuration, zero dependencies. Just one data file. And your code.

    jfest embraces what makes you code faster and safer. Initially inspired by test-driven development and functional programming, it also supports object-oriented programming. Objects are automatically mocked the same way as functions are faked.

    Getting Started

    Install the jfest command

    npm install -g jfest

    Run your first test with jfest

    In your project's root directory:

    user@dev:~/my/javascript/project/$ jfest
    -- watching /home/user/my/javascript/project
    --Cache loaded (function calls)

    Add a $test in a .js file of your choice and save:

    function foo ( bar ) {
        return bar + 1;
    function $test() {

    Check results in console

    Results are displayed in two blocks. First block stands for regression tests from previous saves. Second block is for tests written in $test or $save.

    A test result looks like this:

    foo.js change
    Validating 'foo'
    News[0] Successes[0] Errors[0]
    + foo[null](1) [new] 2
    + foo[null]("a") [new] "a1"
    News[2] Successes[0] Errors[0]
    • + for new
    • = for same result
    • ! for regression
    • function 'name'
    • [this] object in brackets
    • () values of arguments
    • [new] value returned by your current code
    • [old] value expected by the regression checker (omitted if new test case)

    Save results

    Change $test to $save in that file and save:

    function foo ( bar ) {
        return bar + 1;
    function $save() {

    Note that a file jfest.calls has just been created in the running directory. Commit that file along with your code, you and your collaborators will use it later to check they do not break your code.

    Introduce a regression

    Remove any $test or $save from that file, modify your function (e.g. 1 becomes "1") and save:

    function foo ( bar ) {
        return bar + "1";

    Check new results in console

    In the regression tests, we see the function now returns "11" instead of 2:

    foo.js change
    Validating 'foo'
    = foo[null]("a") [new] "a1" [old] "a1"
    ! foo[null](1) [new] "11" [old] 2
    News[0] Successes[1] Errors[1]
    News[0] Successes[0] Errors[0]

    Advanced usage

    Object's methods

    function $test() {
        let foo = {
            bar: 0,
            incr: function () {
                return this.bar++;
    incr[{"bar":1,"incr":"@incr"}]() [new] 0
    incr[{"bar":2,"incr":"@incr"}]() [new] 1
    incr[{"bar":3,"incr":"@incr"}]() [new] 2
    News[3] Successes[0] Errors[0]


    Include as much of the outer scope as needed if your function explicitly uses closure.

    function $test () {
    var x = 1; // Embed this too or you'll get a "ReferenceError"
    function foo () {
        return x; // Explicit closure
    foo[null]() [new] 1
    News[1] Successes[0] Errors[0]


    function foo (f) {
        return f(); // Callback
    function bar () {
        return 1;
    function $test () {
    bar[null]() [new] 1
    + foo[null]("@bar") [new] 1
    News[2] Successes[0] Errors[0]

    Features and current limitations

    Your interactions with jfest are just the $test and $save functions.

    I have made this tool simple on purpose. If jfest cannot test your functions, think first if you could refactor them to be testable.

    $test and $save are run as functions. So you can put in their body whatever code is useful for your tests. Keep in mind that they are temporary functions. After you have used them, you can remove them. Also, their location in your file does not matter. When you use $save, jfest creates a jfest.calls JSON file in the running directory. Once saved, function calls are used to fake similar function calls or check for regressions.

    jfest runs your functions, not your files. To keep it simple, jfest considers all your functions are global.

    So you should:

    • use unique function names across all your project
    • embed in $test or $save body as much of your file as is needed (e.g. outer scope if your function explicitly uses closure)

    Coming features:

    • track changes of this for methods that mutate it
    • give names to test cases (as arguments of $save) - so you get a verbose context of saved function calls
    • do not fake specific functions - some functions are expected to return different results (like random() or Date())

    Any help is welcome!

    If you find a limitation or a bug, please let me know or collaborate :-)


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