Generic Memoize

memoize.js

function memoize( fn ) {
  const map = new Map();
  const key = Symbol('key');

  return function() {
    let item = map;
    let cached = false;

    for ( let i = 0; i < arguments.length; ++i ) {
      let cache = item.get( arguments[ i ] );

      if ( !cache ) {
        item.set( arguments[ i ], new Map() );
        item = item.get( arguments[ i ] );
        cached = false;
        continue;
      }

      item = cache;
      cached = true;
    }

    if ( cached ) {
      return item.get( key );
    }

    let result = fn.apply( this, arguments );

    item.set( key, result );

    return result;
  }
}


function loop( fn, n ) {
  let v;

  for ( let i = 0; i < n; ++i ) {
    v = fn( i );
  }

  return v;
}

let mem = memoize( loop );

console.log( mem( Math.sqrt, 1e9 ) ); // slow
console.log( mem( Math.sqrt, 1e9 ) ); // fast!