JS Simple Assert

assert.css

.assertions {
    list-style-type: none;
    margin: 0;
    padding: 0;
    border-top: 1px solid #000;
    background-color: #fff;
    position: fixed;
    bottom: 0;
    left: 0;
    width: 100%;
    min-height: 100px;
    max-height: 300px;
    overflow: auto;
}

.assertions li {
    padding: 10px 10px;
    border-bottom: 1px solid #ccc;
}

.assertions .error {
    color: red;
    font-weight: bold;
}

.assertions .success {
    color: green;
}

assert.js

/**
 * Usage
 * ------------------------------------------------------------------------
 * assert('Description of assertion', condition, boolWhetherToDebugOnFail);
 * ------------------------------------------------------------------------
 * 
 * Example
 * ------------------------------------------------------------------------
 * assert('String "1" equals numeric 1', '1' == 1);
 * ------------------------------------------------------------------------
 */

(function() {
  var container = document.getElementById('assert-container');
  
  this.assert = function(what, condition, debug) {
    var li = document.createElement('li');
    
    container.appendChild(li);
    
    if (condition) {
      li.classList.add('success');
      what = '✓ ' + what;
    } else {
      li.classList.add('error');
      what = '✘ ' + what;
    }
    
    li.innerText = what;
    container.scrollTop = 999999;
    
    if (!condition) {
      if (debug) {
        debugger;
      } else {
        throw new Error('Execution stopped because of failed assertion');
      }
    }
  };
  
  if (!container) {
    container = document.createElement('ul');
    container.id = 'assert-container';
    container.classList.add('assertions');
    
    document.body.appendChild(container);
  }
})();