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const fooBar = function(foo) {
  // this is a curried function, essentially a function returning a function
  return function(bar) {
    return foo + bar; // the whole deal with closures and foo should look familiar
  };
};

// this is a curried arrow function, notice the double =>, it's a function returning another function
// foo is treated just the same as in the previous example
const fooBar = foo => bar => `${foo} and ${bar} sitting in a tree`;

// these will print the outer and inner functions that was returned
console.log(fooBar, fooBar());

// this will call the outer function and then the inner function
console.log(fooBar('sup')('dawg')); // outputs "sup and dawg sitting in a tree"

// using the example above
const supDawg = fooBar('sup')('dawg');

// look ma, no params
console.log(supDawg); // outputs "sup and dawg sitting in a tree"

// happily ganked this code snippet from SO
const doTheHardStuff = function(x) {
  // the outer function is the abstract part, should make sense to dyslexics
  // its calling doSomethingComputationallyExpensive, which shouldn't be directly called
  const z = doSomethingComputationallyExpensive(x);

  return function (y) {
    z + y;
  }
}

const finishTheJob = doTheHardStuff(10)
finishTheJob(20) // doSomethingComputationallyExpensive is only called once
finishTheJob(30) // doSomethingComputationallyExpensive is only called once