A simple example of currying in functional JavaScript

curried.js

function greetCurried(greeting) {
  return function(name) {
    if (typeof(greeting) !== 'string') {
      return ('Greetings!')
    } else if (typeof(name) !== 'string') {
      return (greeting)
    }

    return (`${greeting}, ${name}`)
  }
}

const greetHello = greetCurried("Hello");
console.log(greetHello('Helen'));
console.log(greetHello(5));

// now it's easy to create a set of functions that share related
// code but provide custom functionality

const greetHi = greetCurried("Hi");
console.log(greetHi('Debbie'));
console.log(greetHi(5));

const greetHowdy = greetCurried("Howdy");
console.log(greetHowdy('Klaudia'));
console.log(greetHowdy(5));

// the curried function even prevents us from creating broken
// versions that incorporate an incorrect type.
const greetWrongType = greetCurried(5)
console.log(greetWrongType('Joey'));
console.log(greetWrongType(5));
console.log(greetWrongType(null));
console.log(greetWrongType(undefined));