doc(Interview Question): Difference between async and defer

Signed-off-by: cankush625 <cankush625@gmail.com>

Author: cankush625

Notes/InterviewQuestions.md

@@ -0,0 +1,26 @@
+## Difference between async and defer in JavaScript
+
+When we include the scripts directly into the HTML, then when HTML parse reach to the line where <br>
+JS scripts are included then it will first stops the HTML parsing and download the script and start executing them. <br>
+After the scripts are fully executed, it will resume the HTML parsing. This will make the HTML parsing slow in browser.
+```
+    <script src="" />
+```
+
+To overcome above issue we use async and defer.
+
+### async attribute
+The `async` attribute will load the scripts and do the HTML parsing in parallel. But as soon as JS scripts are downloaded,
+it will stop the HTML parsing, execute the JS scripts completely and then resume the HTML parsing. <br>
+The `async` attribute does not gurantees the order of execution of the scripts.
+```
+    <script async src="" />
+```
+
+### defer attribute
+The `defer` attribute is best of the both worlds. When we use defer, the downloading of JS scripts and HTML parsing will 
+happen in parallel. Once the HTML parsing is completed, it will execute the JS scripts. <br>
+The `defer` attribute guarantees the order of execution of the scripts.
+```
+    <script defer src="" />
+```