Fibonacci function with memoization [JAVASCRIPT]

fibonacci.js

/// Write a function 'fib(n)' that takes in a number as an argument.
/// The function should return the n-th number of the Fibonacci sequence.

/// The 1st and 2nd number of the sequence is 1.

/// To generate the next number of the sequence, we sum the previous two.
/// fib(n): 1, 1, 2, 3, 5, 8, 13, 21, 34,

const fib = (num, memory = {}) => {
    // Check in memory.
    if (num in memory) return memory[num];

    // Return obvious values to stop recursion.
    if (num <= 1) return num;

    // Store current value in memory.
    memory[num] = fib(num - 1, memory) + fib(num - 2, memory);

    // Return value.
    return memory[num];
}

console.log(fib(50)); // prints => 12586269025