one-paragraph-rule.md

One paragraph rule

There are lots of rules of how to indent and divide rows of code into paragraphs. André Staltz suggested that we limit ourselves to 6 rows of code. I would try to stay in the working memory limit of 7 +/-2 bits according to Miller's law to keep cognitive strain down while reading other people's code. The number 6 still holds well enough, just want to add a reason for it.

One issue that might occurr, is that you need to return a function that has access to a variable on the parent scope:

// the caller needs to call #destroyFn() at some future time (the testrunner ends)
// and #destroyFn() needs to access the variable `server` in the parent scope.
function createServer() {
  const server = http.createServer()
  return function destroyFn() {
    server.close()
  }
}

You have access to the entire parent scope here, which I don't like. And if the destroyFn wants to add some logging message and such, you really want to extract the function. My (yeah, I totally invented it..) proposed solution is to have a higher order function creator that takes the needed variable in as an argument, and thus isolating the scope:

function createDestroyFn(server) {
  return function destroyFn() {
    server.close()
  }
}

function createServer() {
  const server = http.createServer()

  return createDestroyFn(server)
}

Boom! Isolated parent scope, and you get a real life example of a higher order function.

Referenses

• André Staltz video: https://egghead.io/lessons/javascript-refactoring-tip-no-empty-lines-inside-a-function

• Miller's law: https://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two

• My RxJS server project using the higher order function creator with a testrunner (work in progress): https://github.com/marcusnielsen/rxjs-server/blob/master/app/server/index.js#L5