A QnD Solution to GeertVL's Homework ;-)

gistfile1.js

// Part 1.
// Implement a function prototype extension that caches function results for
// the same input arguments of a function with one parameter.
//
// For example:
// Make sin(1) have the result of Math.sin(1), but use a cached value
// for future calls.

Function.prototype.withCaching = function withCaching() {
    if( ! this.cache ) { this.cache = {}; } // lazy initialization of cache
    var method = this;
    return function(input) {
      if( typeof method.cache[input] == "undefined" ) { 
        var result = method.apply(this, [ input ]);
        method.cache[input] = result;
      }
      return method.cache[input];
    };
};

// Part 2.
// Use this new function to refactor the code example.
// Some good test numbers: 524287, 9369319, 2147483647 (all primes)

var isPrime = function( num ) {
  console.log( "looking ..." );
  var prime = num != 1;
  // everything but 1 can be prime
  for ( var i = 2; i < num; i++ ) { 
    if ( num % i == 0 ) { 
      prime = false; 
      break; 
    } 
  } 
  return prime; 
}.withCaching();

console.log( isPrime( 4 ) ); // looking ... false
console.log( isPrime( 4 ) ); // false

console.log( isPrime( 524287 ) ); // looking ... true
console.log( isPrime( 524287 ) ); // true