How to Build an Auto-Expanding Textarea jQuery Plugin, Part 1

Automatically expanding text areas is very popular on sites like Facebook. The height of the text area box expands and shrinks according to the amount of text entered by the user. This has several advantages:

1. Your page design will not be dominated by large text area boxes.
2. Online forms with multiple text areas look shorter and easier to complete.
3. Users can view all text without scrolling.

View the extended text area demonstration...

This three-part tutorial describes how to build an automatically extended text area using HTML and reusable jQuery plug-in. By the end of the third part, you will understand how it works and have code that can be used in your own project.

Demand

Like all good developers, we should fully understand the system requirements:

1. Any text area on any page should be automatically expanded if we need it.
2. However, some text areas may not require automatic expansion.
3. We must be able to specify that the text area height can (a) grow infinitely, or (b) grow within a specific range, for example, use the optimal height between 50 and 200 pixels.
4. Our solution should be reusable on any page we choose.
5. Advanced enhancement technology should be used to ensure that users without JavaScript can still use the standard text area box.
6. This solution should be compatible with IE6, Firefox 2, Opera, Safari, and Chrome.

Proposal

We implement the solution as a jQuery plugin. jQuery is mainly used to handle more mundane aspects such as DOM sniffing and event delegating; you can easily rewrite your code using another framework.

However, how do we know when we should resize the text area and what height to use?

First, we can assign a "keyup" event handler to any text area. This will call a function immediately after the key is pressed and the text is modified.

We can then check the DOM scrollHeight property. This will return the height of the internal scrolling part, i.e. the height of the text entered by the user. If we set the height of the text area to the current scrollHeight value, the scrollbar will become irrelevant. Unfortunately scrollHeight is not W3C recommendation, and we need to overcome some cross-browser inconsistencies:

• In Firefox, Safari, and Chrome, scrollHeight is never smaller than the text area height—even if text is not added. So the box expands, but removing the text won't shrink it. We can solve this problem by temporarily setting the text area height to 0px and then applying the actual scrollHeight value.
• Contrary to the Mozilla scrollHeight documentation, Firefox does not seem to include text area padding, while Safari 4 and Chrome 2 contain it. If we have 2px padding at the top and bottom, scrollHeight will be 4px larger in the WebKit browser and the text area height will grow infinitely. Subtracting the padding will break Firefox, so the easiest solution is to apply the 0px vertical padding to our extended text area elements.
• ScrollHeight is weird in IE and Opera. It usually returns the true height of the internal text, however, setting the text area height to 0px returns an incorrect scrollHeight value. While browser sniffing is terrible, I don't think we have any other options in this case. We must make sure that IE and Opera never set the text area height to 0px.

We also need to consider the scrollbar. By default, most browsers only display text area scroll bars when needed. However, if we set overflow to "auto", the scroll bar will appear when we add a new line and then disappear when we change the height of the text area. Setting overflow to "hidden" will resolve the flashing scrollbar issue, but this must never be applied to non-extended text areas or text areas that have been extended beyond their specified maximum height.

Lastly, browser window resizing can be a problem. Fluid web design can implement text areas with a percentage-based width: resizing windows will resize the box. While we can detect window resize, the event performs poorly in IE and calls handlers quickly. We can solve this problem, but resizing multiple text areas can cause page design to jump and confuse users. So after resizing, we will only adjust the height of the text area after the user switches focus to the box.

There is a lot of content to absorb, and there is more! Make yourself a cup of coffee and get ready for the second part...

• Part 2: Text area initialization
• Part 3: JavaScript Plugin Code
• How to develop jQuery plug-in

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