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Callbacks part I - functions as parameters

This blog post can also be viewed as a screencast, if you rather hear it than read it.

Introduction

One pattern that is used in many places in our code is the concepts of callbacks. They are quite tricky to wrap your head around but once you’ve untangled them they become second nature.

In this little post I wanted to explain a language construct that I’ve seen led to some confusions: passing functions as parameters.

Functions as parameters

JavaScript has a really amazing feature in that you can pass functions as parameters. Like this:

const hiSayer = function () {
  console.log('HI!')
}

function main(fn) {
  console.log('About to call a function passed to me');
  console.log(fn);
  fn();
}

main(hiSayer);

(Save this code as index.js and then run it with. node index.js)

On the first line we are creating an (anonymous) function that writes HI! to the console. We store this function in constant called hiSayer

The main function takes a parameter fn, writes a few statements to the console (the type of the parameter for example… what is that?) and then calls the fn-parameter as a function by using fn();

Finally we call the main function and passing it the hiSayer function as the fn parameter. Hence hiSayer will be called inside main.

Parameter passing

We could make the hiSayer-function a bit more flexible by passing a parameter with the name to who we are going to say hi to. It would look like this:

const hiSayer = function (name) {
  console.log(`HI ${name}!`)
}

function main(nameToGreet, fn) {
  console.log('About to call a function passed to me');
  console.log(fn);
  fn(nameToGreet);
}

main('Marcus', hiSayer);

Now we are passing a string name to the anonymous function. This string is actually a parameter to the main-method too, which means that we can decide (bind) the value of the string when we execute the program on the last line.

Shorter syntax

Actually, this is a bit clunky - let’s shorten it up and take out those extra console.log-statements. Like this:

const hiSayer = name => console.log(`HI ${name}!`);

const main = (nameToGreet, fn) => fn(nameToGreet);

main('Marcus', hiSayer);

Oh! Much sweet! Much nice!

This code is equivalent with the previous version, but shorter, and (objectively) easier to read.

A few things to note:

• hiSayer = function (name) { becomes hiSayer = name =>
  • If there’s more than one parameter you will need parantheis around the parameters (main = (nameToGreet, fn) => for example)
  • A special case is when there’s zero parameters to a function, in which case we also need parentheses const currentDate = () => return Date()
• When there’s only one statement in a function you don’t need curly braces
  • And can bring it up on one line, creating what’s known as a one-liner

Inlining functions

In fact … the function is so short now, that you could ask yourself why we need it at all. Only one line?! Come on! Let’s write it inline, just when we need it. Like this:

const main = (nameToGreet, fn) => fn(nameToGreet);

main('Marcus', name => console.log(`HI ${name}!`));

This is pretty cool! Just as we did with that beautiful name parameter, we can decide (bind) the function just where we need it.

Check the definition of themain-function again:

• it takes a nameToGreet (bound to Marcus when we call it, on the last line) and
• the second parameter fn is a function. Bound to name => console.log(`HI ${name}!`) when we call it.

More than one line

Of course the function doesn’t have to be one line just because we write it inline. But then you need the curly-braces back in - only function that are only one line can skip those

Here’s an example:

const main = (nameToGreet, fn) => fn(nameToGreet);

main('Marcus', name => {
  const upper = name.toUpperCase()
  console.log(`HI ${upper}!`);
});

The function is now wrapped in curly braces. And just as final reminder - that last part could be written, old-school and bloated, like this:

const main = (nameToGreet, fn) => fn(nameToGreet);

main('Marcus', function (name) {
  const upper = name.toUpperCase()
  console.log(`HI ${upper}!`);
});

See that }); at the end there. It’s there because the curly brace ends the definition of the function and the paranthesis ends the parameter list to the main-function. And then the semicolon which is there for our sins.

Summary

Passing functions as parameters is a very powerful construct and can be abbreviated quite a lot. But that it’s short and powerful also makes it a little bit harder to read.

I hope that this post made it a bit clearer.

Now you are well equipped to read the next part of this series - calling back.