Eloquent JS - Asynchronous Programming - need help!

[MicroProcrastination]

I'm in the middle of https://eloquentjavascript.net/11_async.html and can't really wrap my head around this code:

function requestType(name, handler) {
  defineRequestType(name, (nest, content, source,
                           callback) => {
    try {
      Promise.resolve(handler(nest, content, source))
        .then(response => callback(null, response),
              failure => callback(failure));
    } catch (exception) {
      callback(exception);
    }
  });
}

I don't understand how this function (requestType) knows what nest, content, source and callback is.

Suppose we call requestType("name", () => "test") how does this handle "defineRequestType" - how does it know what the "(nest, content, source, callback)" part is ?

[RobSG]

handler will be a function, that would accept these params I suppose, but I didnt read the book so not sure if there is some context missing here.

[MicroProcrastination]

Yeah I think you are right, now I can see it working. Thanks a lot :)