在脚本标记中执行一些javascript后，为什么在html文件中有此文本“ undefined”？

Question

I am executing some java script in an if/else block that prints some tags/content to the html file using document.write() function. This piece of javascript is within the "script" tag just before I end the "body" tag. But I see a very weird behaviour: when my if() condition satisfies, I see some extra text "undefined" in the html. I am not writing this undefined text using JS. At the same time when the code within else {} block gets executed, this undefined text does not appear anymore in my html file. The strcture of my code is as follows:

if(){}
else{
  if(){
    html_text += "<div><p>hello world</p></div>";
    document.write(html_text);
  } else {
    html_text += "<div><p>No hello world</p></div>";
    document.write(html_text);
  }
}

Answer 1

That is because you have never initialized the variable html_text .

<script>
  var html_text = "";
  // Other code
  html_text += "<div><p>hello world</p></div>";

Answer 2

Do everything in right syntax and initial variable before concatenating.

<script>
var html_text = "";

if(your_condition){
     html_text += "<div><p>hello world</p></div>";
     document.write(html_text);
}
else 
{
     html_text += "<div><p>No hello world</p></div>";
     document.write(html_text);
}

</script>