Class CacheFunc is created for caching of functions. The main advantage of instances of this class is high speed of work comparing to simple functions. User has access only to two methods of CacheFunc:

• CacheFunc.do() - executes the function that was cached when the instance of CacheFunc was initialized;
• CacheFunc.clear() - clears the cache;

Cache has these parameters: length of elements, time in cache of each element, priority of crowding out.
Priority of crowding out is calculated by time of execution of function. The more time of execution => the more complex calculations, so
priority of crowding out will be higher.

const cachedFn = new CacheFunc(callback, time, length);

callback - cached function;
time - time in cache of each element in milliseconds;
length - quantity of elements;

const fib = (n) => (n <= 2 ? 1 : fib(n - 1) + fib(n - 2));
const cachedFib = new CacheFunc(fib, 2000, 3);
cachedFib.do(20);
cachedFib.do(20); //from cache
cachedFib.clear() //cache is empty

Class CacheFile is created for caching functions of callback API of library "fs". It makes sense to cache only functions
fs.readFile() and fs.lstat() that is why class CacheFile working correctly only with these methods. The main
advantage of instances of this class is higher speed of work than "fs" methods.
User has access only to these methods of CacheFile:

• CacheFile.prototype[fn.name] - cached function;
• CacheFile.clear() - clears the cache;
• CacheFile.addMethod - adds the cached function;
  Cache has these parameters: size of cache, time in cache of each element, priority of crowding out.
  Priority of crowding out is calculated by size of reading file. The bigger file => the more time needed to read it, so
  priority of crowding out will be higher.

const cachedFS = new CacheFile(callback, time, maxSize);

callback - cached method of "fs";
time - time in cache of each element in milliseconds;
maxSize - maximum size of cache in kilobytes;

const cachedFS = new CacheFile(fs['readFile'], 2000, 10);

cachedFS.readFile('Test-CacheFunc.js', 'utf8', (err, data) => {
  if (err) console.log(err);
  else console.log(data.toString());
  cachedFS.readFile('Test-CacheFile.js', 'utf8', (err, data) => { //from cache
    if (err) console.log(err);
    else console.log(data.toString());
  });
});