async vs defer js

roeib · web-flow · commit ad8e0c80b6a0 · 2020-01-05T10:03:31.000+02:00
diff --git a/README.md b/README.md
@@ -359,3 +359,18 @@ window.addEventListener('contextmenu', ()=>{
 	return false // cancel default menu
 },false)
 ```
+
+
+#  In HTML5, you can tell the browser when to run your JavaScript code
+```javascript
+
+//Without async or defer, browser will run your script immediately, before rendering the elements that's below your script tag.
+<script src="myscript.js"></script>
+
+//With async (asynchronous), browser will continue to load the HTML page and render it while the browser load and execute the script at the same time.
+//Async is more useful when you really don't care when the script loads and nothing else that is user dependent depends upon that script loading.(for scripts likes Google analytics)
+<script async src="myscript.js"></script>
+
+//With defer, browser will run your script when the page finished parsing. (not necessary finishing downloading all image files. 
+<script defer src="myscript.js"></script>
+```
