A one-line comment is written by starting the line with //.

// This is a comment

It's always recommended to clarify your code by commenting:

// A constant representing my phone number
let phoneNumber = 1234567890;

A comment could start at the end of a line:

var weight = 80; // In kilo grams!

And comments could span multiple lines by enclosing it in /** **/:

Multiline comment
A description of the code goes here