Fixes #6

_src/AbsoluteUrl.js

@@ -0,0 +1,5 @@
+const { URL } = require("url");
+
+module.exports = function(url, base) {
+	return (new URL(url, base)).toString()
+};

_src/HtmlToAbsoluteUrls.js

@@ -0,0 +1,20 @@
+const posthtml = require('posthtml');
+const urls = require('posthtml-urls')
+const absoluteUrl = require("./AbsoluteUrl");
+
+module.exports = function(htmlContent, base) {
+  let options = {
+    eachURL: function(url, attr, element) {
+    	// #anchor in-page
+    	if( url.trim().indexOf("#") === 0 ) {
+    		return url;
+    	}
+
+    	return absoluteUrl(url, base);
+    }
+  };
+
+  let modifier = posthtml().use(urls(options));
+
+  return modifier.process(htmlContent);
+};

_src/test/HtmlToAbsoluteUrlsTest.js

@@ -0,0 +1,9 @@
+import test from "ava";
+import htmlToAbsUrls from "../HtmlToAbsoluteUrls.js";
+
+test("Changes a link href", async t => {
+  t.is((await htmlToAbsUrls(`<a href="#testanchor">Hello</a>`, "http://example.com/")).html, `<a href="#testanchor">Hello</a>`);
+  t.is((await htmlToAbsUrls(`<a href="/test.html">Hello</a>`, "http://example.com/")).html, `<a href="http://example.com/test.html">Hello</a>`);
+  t.is((await htmlToAbsUrls(`<img src="/test.png">`, "http://example.com/")).html, `<img src="http://example.com/test.png">`);
+  t.is((await htmlToAbsUrls(`<a href="http://someotherdomain/">Hello</a>`, "http://example.com/")).html, `<a href="http://someotherdomain/">Hello</a>`);
+});