Check maxlength of multiline textbox in JavaScript

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Introduction

ASP.NET provides textbox controls for data inputs and they are used vastly on the websites. TextMode='Multiline' property of textbox is used everywhere to input comments or any large data.

With my program assignment, I came across a problem to set MaxLength for multiline textbox, i.e., after some specified length, the textbox should not accept any characters.

(Note: This solution was only tested & recommended for Internet Explorer (5+), Chrome & Firefox.)

Details

The MaxLength property of textbox control works fine if Multiline mode is not set, but if TextMode='Multiline' is set in aspx page, then this property is of no use and user can input any number of characters inspite of setting maxlength.

Finally, I was able to find a solution and wanted to share it with you all.

This example requires just very basic knowledge of JavaScript, so don’t be scared. I promise this will be one of the easiest JavaScripts that can solve a complex problem.

Add the following to your Multiline box in aspx page:

<asp:TextBox Rows="5" Columns="80" ID="txtCommentsForSearch" 
onkeyDown="return checkTextAreaMaxLength(this,event,'10');" 
onblur="checkLength(this, '10')" TextMode="multiLine" 
CssClass="textArea" runat="server"> </asp:TextBox>        

*txtCommentsForSearch-This is the ASP.NET control that is having multiline property set.

I have used MaxLength='1999', same property you have to use in underlying JavaScript file also. I have also passed this length to the calling JavaScript method, so that in case the MaxLength is not accessible, then it can be picked from parameters of JavaScript method.

In new browser versions, MaxLength of multiline textbox is not accessible in JavaScript, so it's good to assign the required maxlength in JavaScript method calls.

Add the following to your JavaScript file:

// JScript File

function checkTextAreaMaxLength(textBox, e, length) {

            var mLen =  length;
            
            var maxLength = parseInt(mLen);
            if (!checkSpecialKeys(e)) {
                if (textBox.value.length > maxLength - 1) {
                    if (window.event)//IE
                    {
                        e.returnValue = false;
                        return false;
                    }
                    else//Firefox
                        e.preventDefault();
                }
            }
        }

        function checkSpecialKeys(e) {
            
            if (e.keyCode != 9 && e.keyCode != 8 && e.keyCode != 46 && 
            e.keyCode != 35 && e.keyCode != 36 && e.keyCode != 37 && 
            e.keyCode != 38 && e.keyCode != 39 && e.keyCode != 40)
                return false;
            else
                return true;
        }
        function checkLength(textBox, length) {
            var mLen = length;
            
            var maxLength = parseInt(mLen);
            
            if (textBox.value.length > maxLength) {
                textBox.value = textBox.value.substring(0, maxLength);
            }
        }

Note: This implementation is written just to provide a basic and quick help in JavaScript. Nowdays Jquery is preferred and has more easy ways to do things. I myself have moved to Jquery since the past few years. I will post the similar article in Jquery too if anyone is interested.