In other javascript files, i see something that i dont understand

Question

Basically ive completed the javascript course, but when i see other's javascript code, i see many thing that i havent learnt or seen in sololearn javascript. Examples are. let, constant, require

where can i learn this ?

Answer 1

msdn website

Answer 2

Srry It is "mdn" -_-

https://developer.mozilla.org/en-US/docs/Web/JavaScript

Answer 3

msdn website ?