because of best practices

How did you do that???

Not an issue, your code was wrong, but you got lucky and passed it.

Please use markdown formatting to make your code more readable.

I just realized that and I came back here to say so. Well done fast reply, by the way.

Also, I just realized I should have marked that as question.... Oh well.

You could have more than the exact amount of fuel needed to get to the pump.

Please don't put issues on katas if you don't even know what it means.

The tests only work if the test before it works. For example test 2 is called in and is applied if test 1 is passed. Therefore, issue is resolved.

I don't get why the name of the clerk needs to be kept as a variable.

function Clerk(name){
  this.name = name;
  ...
}

I think instead it should be...

function Clerk(){
  ...
}
...
  var vasya = new Clerk();

I also don't get why there are 3 (edit: 2.5???) functions instead of putting all of that in 1 function.

Read what kind of input those methods have.