In redux, we usually comes across connect()() syntax.

Everyone knows that, connect()() function in redux is used for connecting the component with store.

But under the hood, what it exactly means? What can we call such functions ? Is this are normal function like foo() thing ?

Lets see whats its exactly:

What it is knowns as ?
Currying: This methodology or syntax signature of function which takes multiple arguments one at a time is known as 'Curried function' or in short 'Currying'

Is it same as normal / partial function foo()?
Curry: lets you call a function, splitting it in multiple calls, providing one argument per-call.

Partial: lets you call a function, splitting it in multiple calls, providing multiple arguments per-call.

Basically both are same, Currying function helps you to manage code better than partial function and that's the reason on architecture level, usually you will come across curry functions.

Example: Lets do a sum using both partial and curry function:

Partial function:

function sum_partial(a,b,c){
    return a+b+c;
}

Curried Function:

function sum_curried(a) {
    return function (b) {
        return function (c)  {
            return a + b + c
        }
    }
}

Calling partial function:

let res = sum_partial(1, 2, 3);
console.log(res); //6

Calling Curried function:

//Method ONE
let sc1 = sum_curried(1);
let sc2 = sc1(2);
let res2 = sc2(3);
console.log(res2); //6

Short METHOD OR Similar to connect()() in redux

let res3 = sum_curried(1)(2)(3);
console.log(res3); //6

Working JS Fiddle here

For in deep working of connect go here
For more such contents follow @msabir

Cheers!!