a.js

var count = 0;

function increment (amount) {
  count = count + 1;
  return count;
}

increment(1) // 1
increment(1) // 2
increment(1) // 3

b.js

var count = 0;

// now returns a function that becomes called with the count
function increment (amount) {
  return function (lastCount) {
    count = lastCount + amount;
    return count;
  }
}

increment(1)(count) // 1

// composable "add 1" incrementer function
// same as:
const incrementByOne = increment(1);

incrementByOne(count) // 2
incrementByOne(count) // 3
incrementByOne(count) // 4

d.js

function Store (initialState) {
  var state = intiailState;
  
  function transition (transitionFunction) {
    state = transitionFunction(state);
    return state;
  }
  
  return {transition};
}

var countStore = new Store(0);

// no longer set the count, we just return the incremented count
function increment (amount) {
  return function (count) {
    return count + amount;
  }
}

countStore.transition(increment(1)) // 1

// same as:
const incrementByOne = increment(1);

countStore.transition(incrementByOne) // 2
countStore.transition(incrementByOne) // 3
countStore.transition(incrementByOne) // 4

Currying - allows a function to be executed from multiple callsites, where each callsite can provide its own input

W/o currying:

function increment (count, amount) {
  return count + amount;
}

// only callable in one place
increment(1, 1)

W/ currying:

function increment (amount) {
  return function (count) {
    return count + amount;
  }
}

// first call, right here
const incrementByOne = increment(1);

// second call, somewhere else
store.transition(incrementByOne)