cache function result

cache.js

function isPrime( num ) { 
  if ( isPrime.cache[ num ] != null ) 
    return isPrime.cache[ num ]; 
   
  var prime = num != 1; // Everything but 1 can be prime 
  for ( var i = 2; i < num; i++ ) { 
    if ( num % i == 0 ) { 
      prime = false; 
      break; 
    } 
  } 
  
  isPrime.cache[ num ] = prime 
  
  return prime; 
} 
 
isPrime.cache = {}; 
 
assert( isPrime(5), "Make sure the function works, 5 is prime." ); 
assert( isPrime.cache[5], "Make sure the answer is cached." );

////////////// -  element cache

function getElements( name ) { 
  var results; 
 
  if ( getElements.cache[name] ) { 
    results = getElements.cache[name]; 
  } else { 
    results = document.getElementsByTagName(name); 
    getElements.cache[name] = results; 
  } 
 
  return results; 
} 
getElements.cache = {}; 
 
log( "Elements found: ", getElements("pre").length ); 
log( "Cache found: ", getElements.cache.pre.length );