TypeScript

.gitignore

@@ -1 +1,3 @@
-node_modules
+node_modules
+index.js
+index.d.ts

.npmignore

@@ -0,0 +1 @@
+index.ts

index.ts

@@ -0,0 +1,45 @@
+interface URL {
+  type: 'script' | 'style';
+  url: string;
+  options?: {
+    async: boolean;
+    defer: boolean;
+  }
+}
+
+function makeScript(url: string, async: boolean, defer: boolean) {
+  const tag = document.createElement('script')
+  tag.src = url
+  tag.async = async
+  tag.defer = defer
+  return tag
+}
+
+function makeLink(url: string) {
+  const tag = document.createElement('link')
+  tag.rel = 'stylesheet'
+  tag.href = url
+  return tag
+}
+
+export default function (urls: URL[], test: () => boolean, callback: () => void) {
+  let remaining = urls.length
+
+  function maybeCallback () {
+    remaining = --remaining
+    if (remaining < 1) {
+      callback()
+    }
+  }
+
+  if (!test()) {
+    urls.forEach(({ type, url, options = { async: true, defer: true }}) => {
+      const isScript = type === 'script'
+      const tag = isScript ? makeScript(url, options.async, options.defer) : makeLink(url)
+      tag.onload = maybeCallback
+      document.body.appendChild(tag)
+    })
+  } else {
+    callback()
+  }
+}