src/stringProcessing/url.js
function isInternalLink( url, host ) {
	const parsedUrl = urlMethods.parse( url, false, true );
	// Check if the URL starts with a single slash.
	if ( url.indexOf( "//" ) === -1 && url.indexOf( "/" ) === 0 ) {
		return true;
	}

	// Check if the URL starts with a # indicating a fragment.
	if ( url.indexOf( "#" ) === 0 ) {
		return false;
	}

	// No host indicates an internal link.
	if ( ! parsedUrl.host ) {
		return true;
	}

	return parsedUrl.host === host;
}
function isInternalLink( url, host ) {
	const parsedUrl = urlMethods.parse( url, false, true );
	// Check if the URL starts with a single slash.
	if ( url.indexOf( "//" ) === -1 && url.indexOf( "/" ) === 0 ) {
		return true;
	}

	// Check if the URL starts with a # indicating a fragment.
	if ( url.indexOf( "#" ) === 0 ) {
		return false;
	}

	// No host indicates an internal link.
	if ( ! parsedUrl.host ) {
		return true;
	}

	return parsedUrl.host === host;
}
Determine whether a URL is internal.

@param {string} url The URL to test.
@param {string} host The current host.

@returns {boolean} Whether or not the URL is internal.