Hi Sir, do you have a moment to talk about currying? Well, I hope you do, because this is one of the most useful techniques when it comes to Functional Programming.

Ok, but first can you tell me what is currying?

Of course I do Sir, here’s the definition of currying according to Wikipedia:

In mathematics and computer science, currying is the technique of translating the evaluation of a function that takes multiple arguments (or a tuple of arguments) into evaluating a sequence of functions, each with a single argument (partial application).

Don’t worry if you didn’t get it by now, it’s fine, you will surely understand what we’re talking about if you let me give some examples.

Give an example then!

As you wish!

Here we’re going to create a sum function that takes only one argument at a time.

var sum = function firstFunction(a) {
  return function secondFunction(b) {
    return a + b;
  }
}

var onePlus = sum(1);
var result = onePlus(2);

console.log(result);
// --> 3

Here’s what happens when running the code above:

When calling var onePlus = sum(1), you receive the secondFunction(b) and assign it to the onePlus variable.

There’s an important detail to pay attention here: the a variable inside secondFunction gets the value you’ve given as a parameter to the firstFunction.

In a nutshell: imagine onePlus is equivalent to the following:

onePlus = function secondFunction(b) {
  return 1 + b
}

After assigning secondFunction to the onePlus variable we’re calling it and passing 2 as an argument. This means that when we write onePlus(2) we’re invoking secondFunction using 2 as an argument.

In the example above we’ve used currying to add any number to 1, now let’s suppose we need to add any number to 50, how would we do it?

var fiftyPlus = sum(50);

console.log(fiftyPlus(10));
// --> 60

console.log(fiftyPlus(50));
// --> 100

This is awesome, can you give me one more example?

Of course I can.

Here’s the code:

var epicFunction = function(anyName) {
  return function(occupation) {
    return function(currentState) {
      return anyName + ' is the greatest ' + occupation + ' ' + currentState + '.';
    }
  }
}

var aboutLucas = epicFunction('Lucas')('programmer')('alive');
console.log(aboutLucas);
// --> Lucas is the greatest programmer alive.

In the example above we’re calling epicFunction and then getting it’s result (which in this case is another function) and invoking it again passing another parameter. For each call we’ve made the variables inside the innermost function are being assigned the values we pass to the outermost functions.

Just in case you still didn’t get it, I’m gonna show you what happens step by step:

var epicFunction = function(anyName) {
  return function(occupation) {
    return function(currentState) {
      return anyName + ' is the greatest ' + occupation + ' ' + currentState + '.';
    }
  }
}

// First let's call the outermost function
var ryanIsTheGreatest = epicFunction('Ryan Ghosling');

Now ryanIsTheGreatest is the same as:

ryanIsTheGreatest = function(occupation) {
  return function(currentState) {
  	return 'Ryan Ghosling' + ' is the greatest ' + occupation + ' ' + currentState + '.';
  }
}

If we do:

var greatestActor = ryanIsTheGreatest('actor');

The variable greatestActor will be the same as:

greatestActor = function(currentState) {
  	return 'Ryan Ghosling' + ' is the greatest ' + 'actor' + ' ' + currentState + '.';
  }

Finally, when doing:

var greatestActorAlive = greatestActor('alive');

greatestActorAlive will contain the following string: Ryan Ghosling is the greatest actor alive.

In this post you should’ve learned:

What is Currying
How it works
That Ryan Ghosling is the best actor alive

Currying in JavaScript

Ok, but first can you tell me what is currying?

Give an example then!

This is awesome, can you give me one more example?