This article makes more sense if you know about javascript closures. If you want to know about closures please go through this (javascript closures) article.

## What is a curried function?

A curried function is a function that accepts multiple arguments one by one. For example, a curried function of three arguments takes the first argument and returns a function that takes the second argument and returns a function that takes the final argument and returns the final result.

*Example:*

Let's implement a function that takes three arguments and returns the sum at the final.

```
function sum(a){
return function(b){
return function(c){
return a+b+c;
}
}
}
console.log(sum(1)(2)(3));
```

If you are fan of es6 you can write this in single line

```
const sum = a => b => c => a + b + c;
console.log(sum(1)(2)(3));
```

Here, the `sum(1)`

is being called that assigns a as 1 and returns a function which accepts b. Then the returned function will be called with `2`

as argument and that will be assigned to b and another is function will be returned.Again, the returned function will be called with `3`

as it's argument and finally our end result which is the sum of `a,b,c`

will be returned.

## Why currying?

Function currying is very useful in the case of function compositions. Especially in algebra we have function definitions as follow.

```
f: a -> b
g: b -> c
h: a -> c === g(f(x))
```

let's take a simple math example

```
f(x) = x + 1
g(x) = x * 2
Now h(x) = g(f(x)) = g(x+1) = 2 * (x+1) = 2x+2
```

In javascript we can build this composition as follow.

```
const f = x => x + 1; // f(x)
const g = x => x * 2; // g(x)
const compose = (f,g) => x => g(f(x)) // a function that returns a composition of f and g
const h = compose(f,g); // composite function h(x)
```

I hope this adds something to your knowledge and useful β.

Before software can be reusable, it first has to be usable.

β Ralph Johnson

**I am always open to suggestions and accept mistakes. So please leave a comment whatever you feel π€**

Posted on May 24 by:

### Mohana Naga Venkat Sayempu

Full-Stack Developer. In a relationship with web π. Worked for startup companies. Apart from that, I am a good Badminton Player.

## Discussion

Well written! π Will be useful for all the js newbies out there..