Example migrated from Codex:

Example Javascript Handler

The following is one example of what a Javascript live preview file might look like for a custom implementation of the Theme Customizer…

The contents of your theme-customizer.js file might look like this:

/**
 * This file adds some LIVE to the Theme Customizer live preview. To leverage
 * this, set your custom settings to 'postMessage' and then add your handling
 * here. Your javascript should grab settings from customizer controls, and 
 * then make any necessary changes to the page using jQuery.
 */
( function( $ ) {

	// Update the site title in real time...
	wp.customize( 'blogname', function( value ) {
		value.bind( function( newval ) {
			$( '#site-title a' ).html( newval );
		} );
	} );
	
	//Update the site description in real time...
	wp.customize( 'blogdescription', function( value ) {
		value.bind( function( newval ) {
			$( '.site-description' ).html( newval );
		} );
	} );

	//Update site title color in real time...
	wp.customize( 'header_textcolor', function( value ) {
		value.bind( function( newval ) {
			$('#site-title a').css('color', newval );
		} );
	} );

	//Update site background color...
	wp.customize( 'background_color', function( value ) {
		value.bind( function( newval ) {
			$('body').css('background-color', newval );
		} );
	} );
	
	//Update site title color in real time...
	wp.customize( 'mytheme_options[link_textcolor]', function( value ) {
		value.bind( function( newval ) {
			$('a').css('color', newval );
		} );
	} );
	
} )( jQuery );

As you can see from the example above, a single basic handler looks like this…

wp.customize( 'YOUR_SETTING_ID', function( value ) {
	value.bind( function( newval ) {
		//Do stuff (newval variable contains your "new" setting data)
	} );
} );

Note:
Keep in mind that the above will only work if you have set all referenced SETTINGS to ‘transport‘=>’postMessage‘. By default, WordPress uses ‘transport‘=>’refresh‘ for all settings (including the ones that are built in). You can override those defaults using the customize_register action hook.