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Making Curry in Javascript

In this post, we will try to understand Currying in Javascript, how we can make curry function and the reason behind the currying concept.

What is Currying in Javascript?

Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with a single argument and eventually resolve to a value.
Not clear with the definition,right?
Lets understand with example, in which we have written a function to find the volume.

// volume function definition using arrow
const volume = (l, b, h) => (l * b * h); 
// calling volume function
volume(3,2,4);   // 24

Learn about arrow functions,here.
Now here, when we want to call this volume function, we need to pass all the three arguments at one time. Consider a scenario where you get length, breadth and height at different part of the function or javascript file. How can we achieve this? To answer such scenario, we can use currying in javascript.

Making curry function

Let rewrite the above volume function using currying.
const volume = (l) => {
      return (b) => {
              return (h) => l * b * h;
     }
};
// calling this volume function
volume(3)(2)(4);  // 24
Basically, when we call volume by passing first single argument(here 3),it return a function where we pass second single argument(here 2) which return another function where we pass third single argument (here 4) which returns the final result( here 24). 

The word curry has been derived from the mathematician named 'Haskell Curry'. 
In algebra, we can compose a new function from two different function. e.g.
m: a -> b
n: b -> c
the two can be composed to make new function
p: a -> c     by p = m.n i.e. m(n(x))    in algebra

Following this rule in javascript, we can derive
const n = a => a + 5;
const m = a => a * 2;
const p = x => m(n(x));
p(10); //=> 30

Behind the scene

The root cause because of which we are able to make curry function is closure in javascript.
Closure makes currying possible in JavaScript. It’s ability to retain the state of functions already
executed, gives us the ability to create factory functions — functions that can add a specific value to their argument.

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Comment me if any query.
Thank you!!

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