Why is "this" undefined in classes?

Question

I am trying to understand why "this" can be undefined in classes sometimes, and how to fix it. I have read other answers on this but I guess what I'm asking is a solution. I read a lot of open source code because it helps me learn. Every time I see a class, the code just uses this all the way through. Whenever I try to make a class and call this in a callback function, it is undefined. I have used var that = this as a solution but it's causing some issues and I figure this is something I should have figured out a while ago.

Here's an example

class myClass {
     constructor(ping) {
        this.run()
        this.ping = ping
     }

     run() {
        console.log(this.ping)
        //This is defined here
        request({
           url: 'https://test.com',
           method: 'get'
           },
           function (err, res, body) {
             this.ran(body)
            //"ran" is not a function
        })
     }

     ran(body) {
        console.log(body)
     }
 }

Any input is appreciated!