Question

Can someone help me with a javascript function that can highlight text on a web page.
And the requirement is to – highlight only once, not like highlight all occurrences of the text as we do in case of search.

Answer 1

You can use the jquery highlight effect.

But if you are interested in raw javascript code, take a look at what I got
Simply copy paste into an HTML, open the file and click "highlight" – this should highlight the word "fox". Performance wise I think this would do for small text and a single repetition (like you specified)

function highlight(text) {
  var inputText = document.getElementById("inputText");
  var innerHTML = inputText.innerHTML;
  var index = innerHTML.indexOf(text);
  if (index >= 0) { 
   innerHTML = innerHTML.substring(0,index) + "<span class='highlight'>" + innerHTML.substring(index,index+text.length) + "</span>" + innerHTML.substring(index + text.length);
   inputText.innerHTML = innerHTML;
  }
}

.highlight {
  background-color: yellow;
}

<button onclick="highlight('fox')">Highlight</button>

<div id="inputText">
  The fox went over the fence
</div>

Edits:

Using replace

I see this answer gained some popularity, I thought I might add on it.
You can also easily use replace

"the fox jumped over the fence".replace(/fox/,"<span>fox</span>");

Or for multiple occurrences (not relevant for the question, but was asked in comments) you simply add global on the replace regular expression.

"the fox jumped over the other fox".replace(/fox/g,"<span>fox</span>");

Replacing the HTML to the entire web-page

to replace the HTML for an entire web-page, you should refer to innerHTML of the document’s body.

document.body.innerHTML