Calculate big Fibonacci numbers with BigInt and memoization in JavaScript

fibonacci.js

const fibonacci = (n) => {
  // memoization Map: stores all Fibonacci numbers
  const lookup = new Map()

  const fib = (n) => {
    // if the nth number is in the memoization table, return it
    if (lookup.has(n)) return BigInt(lookup.get(n))
    
    // base case: if n is 0 or 1, return n itself
    if (n <= 1) return n
    
    // recursively calculate the nth Fibonacci number from its 2 previous numbers
    const res = fib(n - BigInt(1)) + fib(n - BigInt(2))
    // store the result in the memoization Map
    lookup.set(n, res)
    return res
  }
  
  const result = fib(BigInt(n))
  console.log(`The ${ n }th Fibonacci number is ${ result.toString().length } digits long: \n${ result.toString() }`)
  return result
}

// example usage: Calculate and print the 7855th Fibonacci number
fibonacci(7855)