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HTML

              
                <h3>default UI:</h3>
<ul>
  <li class="pass">this is for what is true</li>
  <li class="fail">this is for what is false</li>
</ul>

<h3>Test:</h3>
<ul id="forTest"></ul>
              
            
!

CSS

              
                .pass:before {
  content: "PASS: ";
  color: green;
  font-weight: bold;
}
.fail:before {
  content: "FAIL: ";
  color: red;
  font-weight: bold;
}

              
            
!

JS

              
                // condition为传入判断 true or false 的语句,
// description 为 string
function assert(condition, description) {
  //找到Ul元素
  var output = document.getElementById("forTest"); 
  //create li 元素
  var li = document.createElement("li"); 
  // 判断condition为何值,true则li为pass的类,false则为fail的类,
  li.className = condition ? "pass" : "fail"; 
  // 为传入的description定义到变量desp,crteate text node
  var des = document.createTextNode(description)
  //des.className = className; 
  
  li.appendChild(des);
  output.appendChild(li);
}

//试一下这个
var a = 5, b, c = 5;
assert((a === b), "'a == b' is true");
assert((a === c), "'a == c' is false");

              
            
!
999px

Console