wp.customize( 'myplugin[border_width]', function( value ) {
 value.bind( function( to ) {
    window.border_style ='';
    window.border_color ='';

    wp.customize( 'myplugin[border_style]', function( value ) {
       value.bind( function( to ) {
          border_style = to;
       } );
    } );
    wp.customize( 'myplugin[border_color]', function( value ) {
       value.bind( function( to ) {
          border_color = to;
       } );
    } );

    if ( !empty(to) ) {
       console.log(to); // works
       console.log(border_style); // empty
       console.log(border_color); // empty
    }
 } );
} );