Use the below links for preparation. Study the topics related to your certification.
What is the Difference between var and let in JavaScript? – https://www.infallibletechie.com/2019/07/what-is-difference-between-var-and-let.html
Difference between null and undefined in JavaScript – https://www.infallibletechie.com/2019/07/difference-between-null-and-undefined.html
Symbol datatype in JavaScript – https://www.infallibletechie.com/2019/07/symbol-datatype-in-javascript.html
https://www.javascriptinstitute.org/javascript-tutorial/
https://www.w3schools.com/js/
