username_exists( string $username )

Source

File: wp-includes/user.php

function username_exists( $username ) {
	$user = get_user_by( 'login', $username );
	if ( $user ) {
		$user_id = $user->ID;
	} else {
		$user_id = false;
	}

	/**
	 * Filters whether the given username exists.
	 *
	 * @since 4.9.0
	 *
	 * @param int|false $user_id  The user ID associated with the username,
	 *                            or false if the username does not exist.
	 * @param string    $username The username to check for existence.
	 */
	return apply_filters( 'username_exists', $user_id, $username );
}