gistfile1.js

// Cache results for functions with one argument

Function.prototype.cached = function(){
  var self = this, cache = {};
  return function(arg){
    if(cache[arg]) return cache[arg];
    return cache[arg] = self(arg);
  }
}

// Same, but with some tracing:

Function.prototype.cached = function(){
  var self = this, cache = {};
  return function(arg){
    if(cache[arg]) {
      console.log('Cache hit for '+arg);
      return cache[arg];
    } else {
      console.log('Cache miss for '+arg);
      return cache[arg] = self(arg);
    }
  }
}

// Let's give it a shot

function isPrime(num) {
  var prime = num != 1;
  for ( var i = 2; i < num; i++ ) {
    if ( num % i == 0 ) { 
      prime = false; 
      break; 
    }
  }
  return prime;
}

isPrime = isPrime.cached();