common: add combineURLs() helper

This combines URLs similar to an URL join, but assumes the
base URL is the root path instead of the host.

This is useful for combining API paths with API server entry points.

packages/common/index.js

@@ -16,4 +16,5 @@ export {Spinner};
 export {InlineNotification};
 export {Translated};
 export * from './src/logger.js';
+export * from './src/utils.js';
 export {AdapterLitElement};

packages/common/src/utils.js

+/**
+ * Appends the second relative or absolute URL by treating
+ * the base URL as the root path. Unlike normal URL join which
+ * treats the host as root path.
+ * 
+ * http://example.com/foo + bar -> http://example.com/foo/bar
+ * http://example.com/foo/ + /bar -> http://example.com/foo/bar
+ *
+ * @param {string} baseURL The bas URL
+ * @param {string} addedURL The URL to append ot the baseURL
+ */
+export const combineURLs = (baseURL, addedURL) => {
+    if(!baseURL.endsWith('/')) {
+        baseURL += '/';
+    }
+    return new URL(addedURL.replace(/^\/+/, ''), baseURL).href;
+};
\ No newline at end of file

packages/common/test/unit.js

 import {expect, assert} from '@esm-bundle/chai';
 import * as utils from '../utils';
 import * as styles from '../styles';
+import {combineURLs} from '../';
 import '../jsonld.js';
 
 suite('utils', () => {
@@ -44,4 +45,18 @@ suite('utils', () => {
     test('getThemeCSS', () => {
         styles.getThemeCSS();
     });
+
+    test('combineURLs', () => {
+        assert.equal(combineURLs('http://example.org/foo', 'bar'), "http://example.org/foo/bar");
+        assert.equal(combineURLs('http://example.org/foo', '/bar'), "http://example.org/foo/bar");
+        assert.equal(combineURLs('http://example.org/foo/', '/bar/'), "http://example.org/foo/bar/");
+        assert.equal(combineURLs('http://example.org', '/bar'), "http://example.org/bar");
+        assert.equal(combineURLs('http://example.org', 'bar/'), "http://example.org/bar/");
+        assert.equal(combineURLs('http://example.org', ''), "http://example.org/");
+        assert.equal(combineURLs('http://example.org/bla', ''), "http://example.org/bla/");
+        assert.equal(combineURLs('http://example.org/bla/', ''), "http://example.org/bla/");
+        assert.equal(combineURLs('http://example.org', 'http://other.com'), "http://other.com/");
+        assert.equal(combineURLs('http://example.org', 'http://other.com/test'), "http://other.com/test");
+        assert.equal(combineURLs('http://example.org', 'http://other.com/test/'), "http://other.com/test/");
+    });
 });