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The Not-So-Scary Explanation Of Currying In Javascript

loujaybee profile image Lou β€” Cloud Native Software Engineer ・2 min read

If I have a pet hate with the developer community it's that a lot of concepts in technology are simply explained with needless complexity. "Currying" I've found to be one of these ideas. Currying is in it's simplest form pretty simple. Let's take a look at it.

To show you how and why to use currying let us look at an example.

Currying Explained Simply

Whilst refactoring code I saw a good chance to use Currying.

The code I saw looked like this simplified example:

var contains = function(one, two){
  // Computation
};

var one = {};

contains(one, "Some string")
contains(one, "A different string");
contains(one, "Another string");

Smelling the code

When writing / reviewing / refactoring code we often look for what is known as a "code smells". A code smell is similar to an anti-pattern. It should stop you in your tracks and make you say "hmmm".

This code gave me a strong whiff of a code smell.

Why? The repetition of the code contains(one.

A potential upgrade

I immediately started looking for ways to shorten this code.

A possibility to improve would be to hardcoded the one value inside the contains function. Like so:

var contains = function(two){
  var one = value;

  // Computation
};

contains("Some string")
contains("A different string");
contains("Another string");

This does stop the repetition of always passing the first value. but it reduces the reuse of the contains code.

It couples it to the implementation of the one value.

So how do we reduce this repetition without creating tight coupling?

Enter Currying

In order to prevent the repetition of the first variable we can leverage javascripts closure functionality.

var contains = function(one){
  return function(two) {
    // Computation
  }
};
var boundContains = contains(one);

boundContains("Some string")

boundContains("A different string");

boundContains("Another string");

What's happening here?

  • The contains function is called once with the variable one.
  • The Javascript closure then saves the reference to the one variable in a closure
  • The bind contains method is now equivalent to the returned function from inside contains
  • We can now use the bindContains method, without always passing the one parameter.
  • If the one value changes, now we have only one reference to update.

Conclusion

This is a simplified example. But hopefully you can see how currying can be used to DRY up our code.

It doesn't have to be so esoteric either.

You can even take currying further by creating a curry method, but that's a lesson for another day!

Posted on Sep 1 '17 by:

loujaybee profile

Lou β€” Cloud Native Software Engineer

@loujaybee

I write stuff on Cloud Engineering @ thedevcoach.co.uk. I made splitoo.com. Also likes: πŸ‹οΈβ€β™‚οΈπŸŽΈπŸš΄πŸ»β€β™‚οΈπŸ

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