JavaScript Tip of the Day

gistfile1.js

var counter = 0;

var n_calls = function () {
  console.log("This function was called ", ++counter, " times");
};

n_calls(); // This function was called 1 times
n_calls(); // This function was called 2 times
n_calls(); // This function was called 3 times

// evil hacker code:
counter = NaN;

n_calls(); // This function was called  NaN  times

// NOW WITH CLOSURE:

var n_calls = function () {
  var counter = 0;

  return function () {
    console.log("This function was called ", ++counter, " times");
  };
}();


n_calls(); // This function was called 1 times
n_calls(); // This function was called 2 times
n_calls(); // This function was called 3 times

// evil hacker code:
counter = NaN;

n_calls(); // This function was called 4 times
// \o/

// Example 2: avoid reevaluation

// regular fibo without cache

var calls = 0;

var fibo = function(n) {
  ++calls;

  if (n == 0 || n == 1) { return n; }

  return fibo(n - 1) + fibo(n - 2);
}

calls = 0;
console.log(fibo(10)); // 55
console.log(calls); // 177

calls = 0;
console.log(fibo(10)); // 55
console.log(calls); // 177

calls = 0;
console.log(fibo(20)); // 6765
console.log(calls); // 21891

// with awesome hawt closure cache

var calls = 0;

var fibo = function() {
  var cache = [0, 1];

  return function(n) {
    ++calls;

    if (typeof cache[n] !== 'number') {
      return (cache[n] = fibo(n - 1) + fibo(n - 2));
    } else
      return cache[n];
  };
}();

calls = 0;
console.log(fibo(10)); // 55
console.log(calls); // 19

calls = 0;
console.log(fibo(10)); // 55
console.log(calls); // 1

// without resetting the env (reloading the page)

console.log(fibo(20)); // 6765
console.log(calls); // 21

// resetting the env

console.log(fibo(20)); // 6765
console.log(calls); // 21

// resetting the env

console.log(fibo(100)); // 354224848179262000000
console.log(calls); // 199 <- 22 more than regular fibo(10)

// HOT!