curried-function.md

Is a technique for converting function calls with N arguments into chains of N function calls with a single argument for each function call?

Currying always returns another function with only one argument until all of the arguments have been applied. So, we just keep calling the returned function until we’ve exhausted all the arguments and the final value gets returned.

Example

const add = (x, y) => x + y
add(2, 3) // => 5

And now, below is the same function in curried form.

const add = x => y => x + y

Same code without arrow syntax

const add = function(x) {
  return function(y) {
    return x + y
  }
}

Calling curried functions

So in order to use our curried function, we have to call it a bit differently …

add(2)(3) // returns 5

Focus on return

It might help to visualize it another way. We know that arrow functions work like this – let's pay particular attention to the return value.

const f = someParam => returnValue

So our add function returns a function – we can use parentheses for added clarity.

const add = x => y => x + y

More than two arrow functions can be sequenced, if necessary -

const three = a => b => c => a + b + c

const four = a => b => c => d => a + b + c + d

three(1)(2)(3) // 6

four(1)(2)(3)(4) // 10

In other words add of some number returns a function

Further Reading

stackoverflow.com/questions/32782922/what-do-multiple-arrow-functions-mean-in-javascript/32784025