functions.php

<?php

add_action( 'wp_ajax_my_request', function () {
    ob_start();

    echo get_theme_mod( 'position', 'top' );

    wp_send_json_success( ob_get_clean() );

    wp_die();
} );

add_action( 'wp_enqueue_scripts', function() {
    wp_enqueue_script(
    	'my-request',
    	get_theme_file_uri( 'request.js' ),
    	array( 'jquery' ),
    	null,
    	true
    );

    wp_localize_script(
        'my-request',
        'myArgs',
        array(
          'url'    => admin_url( 'admin-ajax.php' ),
          'action' => 'my_request',
        )
    );
} );

add_action( 'customize_register', function( $wp_customize ) {
    $wp_customize->add_section( 'position', array(
        'title'    => 'Position',
        'priority' => 1,
    ) );

    $wp_customize->add_setting( 'position', array(
        'default' => 'top',
    ) );

    $wp_customize->add_control( 'position', array(
        'label'   => 'position',
        'section' => 'position',
        'type'    => 'select',
        'choices' => array(
          'top'    => 'Top',
          'right'  => 'Right',
          'bottom' => 'Bottom',
          'left'   => 'Left',
        ),
    ) );
} );

request.js

(function( jQuery ) {

  'use strict';

  var data = {
    action: myArgs.action,
  }

  function request() {
    jQuery.post( myArgs.url, data, function( response ) {
      console.log( response );
    } );
  }

  document.addEventListener( 'DOMContentLoaded', function() {
    request();
  } );

})( jQuery );