bench.html

<script src='bench.js'></script>

bench.js

if (undefined === window.testMeTests)
    window.testMeTests = [];

Function.prototype.testMe = function (input, expected) {
    testMeTests.push({
        unit: this,
        input: input,
        expected: expected,
    });
    return this;
}

window.setInterval(function() {
    var results = window.testMeTests.map(function(test) {
        var result = test.unit.apply(null, test.input)
        return Object.assign({}, test, {
            result: result,
            ok: result === test.expected
        });
    });
    document.open();
    results.forEach(function (result) {
        document.write('<pre style=\'color:'+(result.ok?'green':'red')+'\'>');
        document.write(JSON.stringify(result, null, ' '));
        document.write('</pre>');
    });

}, 300)

// Open DevTools and modify this function...
var add = function(a,b) {
    return a + b;
}
    .testMe([1, 2], 3)
    .testMe([1, -1], 0);