Source

File: wp-includes/rest-api.php

function get_rest_url( $blog_id = null, $path = '/', $scheme = 'rest' ) {
	if ( empty( $path ) ) {
		$path = '/';
	}

	$path = '/' . ltrim( $path, '/' );

	if ( is_multisite() && get_blog_option( $blog_id, 'permalink_structure' ) || get_option( 'permalink_structure' ) ) {
		global $wp_rewrite;

		if ( $wp_rewrite->using_index_permalinks() ) {
			$url = get_home_url( $blog_id, $wp_rewrite->index . '/' . rest_get_url_prefix(), $scheme );
		} else {
			$url = get_home_url( $blog_id, rest_get_url_prefix(), $scheme );
		}

		$url .= $path;
	} else {
		$url = trailingslashit( get_home_url( $blog_id, '', $scheme ) );
		// nginx only allows HTTP/1.0 methods when redirecting from / to /index.php.
		// To work around this, we manually add index.php to the URL, avoiding the redirect.
		if ( 'index.php' !== substr( $url, 9 ) ) {
			$url .= 'index.php';
		}

		$url = add_query_arg( 'rest_route', $path, $url );
	}

	if ( is_ssl() && isset( $_SERVER['SERVER_NAME'] ) ) {
		// If the current host is the same as the REST URL host, force the REST URL scheme to HTTPS.
		if ( parse_url( get_home_url( $blog_id ), PHP_URL_HOST ) === $_SERVER['SERVER_NAME'] ) {
			$url = set_url_scheme( $url, 'https' );
		}
	}

	if ( is_admin() && force_ssl_admin() ) {
		/*
		 * In this situation the home URL may be http:, and `is_ssl()` may be false,
		 * but the admin is served over https: (one way or another), so REST API usage
		 * will be blocked by browsers unless it is also served over HTTPS.
		 */
		$url = set_url_scheme( $url, 'https' );
	}

	/**
	 * Filters the REST URL.
	 *
	 * Use this filter to adjust the url returned by the get_rest_url() function.
	 *
	 * @since 4.4.0
	 *
	 * @param string $url     REST URL.
	 * @param string $path    REST route.
	 * @param int    $blog_id Blog ID.
	 * @param string $scheme  Sanitization scheme.
	 */
	return apply_filters( 'rest_url', $url, $path, $blog_id, $scheme );
}