partial.js

// `num` now exists within the closure created by
// add, and will be available in any returned function
function add(num) {
  
  // this returned function will wait for execution with an
  // argument to be provided later
  return function partial(otherNum){
   
    // return the sum of the two arguments
    return num + otherNum
  }
}

const stepOne = add(3)
const stepTwo = stepOne(5)
console.log(stepTwo)

const combo = add(5)(10)
console.log(combo)