I was recently asked to create a textbox that would vertically expand depending on how much was written in it. This style of text box can be seen in the Facebook news feed for writing comments under peoples feed items. When I receive this type of request, I usually begin by researching what existing systems and plug-ins are out there, investigate the different approaches people take and then write my own based on which way I think best.

Firstly, a textbox (input type=text) is the wrong element to be using. It cannot be any larger than a single line, so a text area element needs to be used. As a text area element will not automatically expand by itself, the general approach to this problem is to use a hidden ‘staging’ element placed way off the viewable screen. The text entered into the text area is copied across to the staging element, which expands as needed and the resulting height used to resize the text area. It is not a difficult thing to achieve with jQuery and the basic structure of the plug-in can be made in relatively few lines of code.

There are a few things to watch out for however. Firstly, because we are taking the contents of a text area and placing it in a div, we need to watch out for special characters that need encoding before they will render correctly. Primarily the newline character but we also have to watch out for the other special characters that can cause issues. Nothing that cannot be fixed with regular expressions. The other thing that we need to be careful of is the CSS styles that have been applied to the text area. We require that the hidden element perfectly mimic the text area and that the text wraps at the same point. If it does not, we will incorrectly predict when the text area needs to grow. The font size and family, line height and padding need to be copied.

This would normally be the point that I post the code, but after I started writing the plug-in, I stumbled across an existing one that I could just not improve upon and decided to use it in it’s entirety. Its by a guy called Jason Frame and his plug-in can be found on github. For my particular implementation, I added the facility to have watermark text in it by utilising the great plugin over on digital bush and I also added some text parsing requirements that were required for the particular project.

Now for a word of warning. There are some problems with this approach, which I have not managed to solve (and incidentally, nor have Facebook). The text area element renders it’s content slightly differently in every browser. In the hidden element, the text will sit up directly against the edge of the div but in a text area it may not. I suspect this gap also varies from operating system to operating system but as far as I can tell, you cannot eliminate it with styles. The consequence of this is that predicting precisely when the text area needs resizing becomes very difficult. The only thing to do is to accommodate this margin of error. This can be done by making the text area display slightly taller than a single line and to always have an extra line available. That way, if things go slightly off, it is still usable. You can see this problem if you put a significant amount of dummy text into the text area and watch when it expands as you type. Instead of expanding when you reach a new line it happens at a different place. If any one manages to figure out how to completely eliminate this problem I am eager to hear how.