WP Upload Image from Front-End

function.php

<?php

/**
 * Create the image attachment and return the new media upload id.
 *
 * @param $fileObject
 * @return false|int|WP_Error
 * @see http://codex.wordpress.org/Function_Reference/wp_insert_attachment#Example
 * @since 1.0.0
 */
function xc_create_image_id($fileObject)
{

  // Cache info on the wp uploads dir
  $wp_upload_dir = wp_upload_dir();
  // $wp_upload_dir['path'] is the full server path to wp-content/uploads/2017/05, for multisite works good as well
  // $wp_upload_dir['url'] the absolute URL to the same folder, actually we do not need it, just to show the link to file
  $i = 1; // number of tries when the file with the same name is already exists

  $new_file_path = $wp_upload_dir['path'] . '/' . $fileObject['name'];
  $new_file_mime = mime_content_type($fileObject['tmp_name']);

  if (empty($fileObject))
    die('File is not selected.');

  if ($fileObject['error'])
    die($fileObject['error']);

  if ($fileObject['size'] > wp_max_upload_size())
    die('It is too large than expected.');

  if (!in_array($new_file_mime, get_allowed_mime_types()))
    die('WordPress doesn\'t allow this type of uploads.');

  while (file_exists($new_file_path)) {
    $i++;
    $new_file_path = $wp_upload_dir['path'] . '/' . $i . '_' . $fileObject['name'];
  }

  $upload_id = false;
  if (move_uploaded_file($fileObject['tmp_name'], $new_file_path)) {

    $upload_id = wp_insert_attachment(array(
      'guid' => $new_file_path,
      'post_mime_type' => $new_file_mime,
      'post_title' => preg_replace('/\.[^.]+$/', '', $fileObject['name']),
      'post_content' => '',
      'post_status' => 'inherit'
    ), $new_file_path);

    // wp_generate_attachment_metadata() won't work if you do not include this file
    require_once(ABSPATH . 'wp-admin/includes/image.php');

    // Generate and save the attachment metas into the database
    wp_update_attachment_metadata($upload_id, wp_generate_attachment_metadata($upload_id, $new_file_path));

  }

  //Error check
  if (!is_wp_error($upload_id)) {
    //Generate wp attachment meta data
    if (file_exists(ABSPATH . 'wp-admin/includes/image.php') && file_exists(ABSPATH . 'wp-admin/includes/media.php')) {
      require_once(ABSPATH . 'wp-admin/includes/image.php');
      require_once(ABSPATH . 'wp-admin/includes/media.php');
      $attach_data = wp_generate_attachment_metadata($upload_id, $new_file_path);
      wp_update_attachment_metadata($upload_id, $attach_data);
    } // end if file exists check
  } // end if error check

  return $upload_id;
}


/**
  * CALL ON AJAX ACTION
    $avatarURL = false;
		if ( ! empty( $_FILES["avatar"]["name"] ) ) {
			$attach_id = xc_create_image_id( $_FILES["avatar"] );
			update_field( 'avatar', $attach_id, $post_id );
			$avatarURL = wp_get_attachment_url( $attach_id );
		}
 **/