memoizationPromises.js

// Higher order memoize function
// fn returns a promise (not synchronous)
function memoize (fn) {
    const lookup = {}
    const memoized = function () {
        const args = Array.from(arguments)
        if (args in lookup) {
            return lookup[args]
        }
        return fn.apply(this, args).then(val => {
            lookup[args] = val
            return val
        })
    }
    memoized.getLookup = () => lookup
    return memoized
}

// Original fibonacci function
function fibonacci (n) {
    if (n === 0 || n === 1) {
        return Promise.resolve(n)
    }
    return Promise.all([fibonacci(n - 1), fibonacci(n - 2)]).then(results => results.reduce((a, b) => a + b, 0))
}

// Memoized fibonacci function
const memoizedFibonacci = memoize(fibonacci)

// Redefine fibonacci to recursively use the memoized version (cache intermediate results)
function fibonacci (n) {
    if (n === 0 || n === 1) {
        return Promise.resolve(n)
    }
    return Promise.all([memoizedFibonacci(n - 1), memoizedFibonacci(n - 2)]).then(results => results.reduce((a, b) => a + b, 0))
}

// See how the cached results grow with memoizedFibonacci.getLookup()
// e.g.,
// memoizedFibonacci.getLookup() --> {}
// fibonacci(5).then(val => console.log(val)) --> 5
// memoizedFibonacci.getLookup() --> {0: 0, 1: 1, 2: 1, 3: 2, 4: 3}