cacheSavings.js

/*
Cache Savings
Caching is used in software engineering to temporarily store the result of 
an "expensive" operation (takes a lot of time or memory). If that result 
is needed again in the near future, it can be retrieved from the cache 
instead of calculating it again.

Write a function, cacheSavings, that takes a callback and returns a new 
function that takes one argument. When the new function is called for the 
first time, it should call the callback with the argument and save the 
result of the function call. If the new function is called again with the 
same argument, the new function should retrieve the stored value instead 
of calling the callback again!
*/

function cacheSavings2 (callback) {
    // receives a callback function
    return function (arg) {
        // call the callback passing the new arg and returning the result
        let result = callback(arg); 
        return result;
    }
}

// another way of doing this and shortening it
let cacheSavings = cb => arg => cb(arg);

// TEST CONDITIONS
function adds10(num) {
  let start = Date.now();
  let end = Date.now();
  // pause for 3 seconds, to make the function expensive!
  while (end - start < 3000) end = Date.now();
  return num + 10;
}
let cachedAdds10 = cacheSavings(adds10);
console.log(cachedAdds10(20)); // => returns 30, takes 3 seconds!
console.log(cachedAdds10(0)); // returns 10, takes 3 seconds!
console.log(cachedAdds10(20)); // => returns 30, takes no time at all!