Teacher, after learning jQuery, is it your turn to take this course?

I originally thought that I should take the course on the front-end practical native layout next, but it prompted me to take the DOM course.

The software used in this course has not been taught before, so I explored it. Okay, so before declaring variables in js, I only talked about using var. In this class, let is used to declare variables. What is the difference between the two? Or did I take the courses in the wrong order?