WordPress/wp-content/themes/twentyfifteen/js/customize-preview.js

/**
 * Live-update changed settings in real time in the Customizer preview. 
 */

( function( $ ) {
	// Site title and description.
	wp.customize( 'blogname', function( value ) {
		value.bind( function( to ) {
			$( '.site-title a' ).text( to );
		} );
	} );
	wp.customize( 'blogdescription', function( value ) {
		value.bind( function( to ) {
			$( '.site-description' ).text( to );
		} );
	} );
} )( jQuery );