wiki.techinc.nl/resources/jquery/jquery.byteLimit.js

/**
 * jQuery byteLimit
 *
 * @author Jan Paul Posma
 */
( function( $ ) {

	/**
	 * Enforces a byte limit to a textbox, so that UTF-8 entries are counted as well, when, for example,
	 * a databae field has a byte limit rather than a character limit.
	 * Plugin rationale: Browser has native maxlength for number of characters, this plugin exists to
	 * limit number of bytes instead.
	 *
	 * Can be called with a custom limit (to use that limit instead of the maxlength attribute value),
	 * a filter function (in case the limit should apply to something other than the exact input value),
	 * or both. Order of arguments is important!
	 *
	 * @context {jQuery} Instance of jQuery for one or more input elements
	 * @param limit {Number} (optional) Limit to enforce, fallsback to maxLength-attribute,
	 * called with fetched value as argument.
	 * @param fn {Function} (optional) Function to call on the input string before assessing the length
	 * @return {jQuery} The context
	 */
	$.fn.byteLimit = function( limit, fn ) {
		// If the first argument is the function,
		// set fn to the first argument's value and ignore the second argument.
		if ( $.isFunction( limit ) ) {
			fn = limit;
			limit = undefined;
		}

		// Default limit to current attribute value
		if ( limit === undefined ) {
			limit = this.attr( 'maxLength' );

		// If limit passed, update/set attribute value instead
		} else {
			this.attr( 'maxLength', limit );
		}

		// Nothing passed and/or empty attribute, return without binding an event.
		if ( limit === undefined ) {
			return this;
		}

		// Save function for reference
		this.data( 'byteLimit-callback', fn );

		// We've got something, go for it:
		return this.keypress( function( e ) {
			// First check to see if this is actually a character key
			// being pressed.
			// Based on key-event info from http://unixpapa.com/js/key.html
			// jQuery should also normalize e.which to be consistent cross-browser,
			// however the same check is still needed regardless of jQuery.
	
			// Note: At the moment, for some older opera versions (~< 10.5)
			// some special keys won't be recognized (aka left arrow key).
			// Backspace will be, so not big issue.
	
			if ( e.which === 0 || e.charCode === 0 || e.which === 8 ||
				e.ctrlKey || e.altKey || e.metaKey )
			{
				return true; //a special key (backspace, etc) so don't interfere.
			}

			var	val = fn !== undefined ? fn( $( this ).val() ): $( this ).val(),
				len = $.byteLength( val ),
				// Note that keypress returns a character code point, not a keycode.
				// However, this may not be super reliable depending on how keys come in...
				charLen = $.byteLength( String.fromCharCode( e.which ) );

			if ( ( len + charLen ) > limit ) {
				e.preventDefault();
			}
		});
	};

} )( jQuery );