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Posted 26 Jul 2019

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HTML Line Numbering using textarea

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26 Jul 2019CPOL2 min read
HTML line numbering using textarea


This is a simple solution for numbering lines in a text in HTML page (i.e., code writing). Since, to my knowledge, there is no HTML element that is offering this possibility by default, I created a small and simple code that perfectly and effectively does the trick, even for very long texts.

Image 1

It uses 2 textarea elements, the right one is for writing or pasting text, and the left one is readonly and populated by JavaScript, it shows the lines of the text. The 2 textarea elements are scroll-synced in the JS code. There is also another element underneath that is showing the current position/selection in the text.

Using the Code

The HTML part is very short, there are 2 textarea elements, wrapped together in a div; the textarea for writing is wrapped together with input element for showing current position/selection in a span - this is purely for easier access to these elements from within the JavaScript code.

<body onload="initialize()" onresize="onresize_sub()">
    <h1>Line numbering using &lt;textarea&gt;</h1>
        <textarea class="rownr" rows="20" 
        cols="3" value="1" readonly></textarea>
            <textarea class="txt" rows="20" 
            cols="150" nowrap="nowrap" wrap="off"
            autocomplete="off" autocorrect="off" 
            autocapitalize="off" spellcheck="false"
            onkeyup="keyup(this,event)" oninput="input_changed(this)" 
            <label>Current position: 
            </label><input id="sel_in" style="border-style:none" readonly>

The CSS is also very simple, some coloring and text alignment:

   .rownr {overflow-y: hidden; background-color: rgb(105,105,105); color: white; 
           text-align: right; vertical-align:top}
   .txt {width: 95%; overflow-x: scroll}

The main textarea uses event handlers for:

  • oninput (for populating line numbers)
  • onscroll (for scroll syncing)
  • onclick and onkeyup events (for reading current position/selection)

Oninput Event Handler

When some text is entered into the textarea, function input_changed is called:

function input_changed(obj_txt)
        obj_rownr = obj_txt.parentElement.parentElement.getElementsByTagName('textarea')[0];
        cntline = count_lines(obj_txt.value);
        if(cntline == 0) cntline = 1;
        tmp_arr = obj_rownr.value.split('\n');
        cntline_old = parseInt(tmp_arr[tmp_arr.length - 1], 10);
        // if there was a change in line count
        if(cntline != cntline_old)
            obj_rownr.cols = cntline.toString().length; // new width of txt_rownr
            populate_rownr(obj_rownr, cntline);

This function is calling another function count_lines to count the lines in the text (by splitting text by \n and counts the elements of the resulting array). Then, if there was indeed a change in the number of lines, rownr textarea is repopulated.

Onscroll Event Handler

The onscroll event handler ( scroll_changed ) is, basically, reading the scrollTop attribute of one textarea, and sets this attribute for the other textarea. (This function can also work with any 2 scrollable HTML elements.)

function scroll_changed(obj_txt)
        obj_rownr = obj_txt.parentElement.parentElement.getElementsByTagName('textarea')[0];
function scrollsync(obj1, obj2)
        // scroll text in object id1 the same as object id2
        obj2.scrollTop = obj1.scrollTop;

Onclick and onkeyup Event Handlers

When the textarea is clicked, selectionchanged function is called.

function selectionchanged(obj)
    var substr = obj.value.substring(0,obj.selectionStart).split('\n');
    var row = substr.length;
    var col = substr[substr.length-1].length;
    var tmpstr = '(' + row.toString() + ',' + col.toString() + ')';
    // if selection spans over
    if(obj.selectionStart != obj.selectionEnd)
        substr = obj.value.substring(obj.selectionStart, obj.selectionEnd).split('\n');
        row += substr.length - 1;
        col = substr[substr.length-1].length;
        tmpstr += ' - (' + row.toString() + ',' + col.toString() + ')';
    obj.parentElement.getElementsByTagName('input')[0].value = tmpstr;

This simple function uses the selectionStart and selectionEnd attributes of textarea objects (also works for some other HTML elements) to calculate the current line and column where the snippet is located.

In order to handle also the arrow keys on the keyboard, plus the Home, End, Page Up and Page Down keys, there is also a handler function for keyUp event. This function is doing nothing but calling the selectionchanged function if any of these keys are pressed.

function keyup(obj, e)
        if(e.keyCode >= 33 && e.keyCode <= 40) // arrows ; home ; end ; page up/down
            selectionchanged(obj, e.keyCode);

Image 2


  • 26th July, 2019: Initial version


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Marijan Nikic
User Interface Analyst Raiffeisenbank Austria
Croatia Croatia
I acquired Masters degree in computing science at the Faculty of Electrical Engineering and Computing in Zagreb, Croatia in 2009. Following my studies, I got a job in a Croatian branch of Austrian-based CEE Raiffeisen Bank as an MIS (Management information system) analyst.
I have been working there since 2010, as an IT expert within the Controlling department, maintaining the Oracle's OFSA system, underlying interfaces and databases.
Throughout that time, I have worked with several different technologies, which include SQL & PL/SQL (mostly), postgres, Cognos BI, Apparo, Datastage, ODI, Jenkins, Qlik, ...
I am doing a lot of automation with scripting in batch / shell and VBscript (mostly) - data analysis and processing, automated DB imports and exports, Jenkins automation etc.
Privately, I was mostly doing Windows Forms and Console app tools in Visual Studio, C#.

Comments and Discussions

Questionwhere's the populateRownR function? Pin
Karan Parsnani9-May-20 4:14
MemberKaran Parsnani9-May-20 4:14 

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