Memoization for performance in functional Javascript

Anton Ioffe - September 22nd 2023 - 11 minutes read

As a great JavaScript developer, your job is not only about creating smooth and responsive applications, but also ensuring that they run as efficiently as possible. One powerful technique for boosting your applications’ performance is employing memoization, but it’s one that’s often overlooked or misunderstood. This comprehensive guide aims to shed light on the intricacies of memoization in JavaScript, turning this sometimes complex concept into an indispensable tool in your developer’s arsenal.

Through the article, you'll gain not only a theoretical understanding of memoization but also its practical application aspects. We'll provide step-by-step guides to implementing memoization in your code and walk you through common mistakes often made during its implementation. We'll then navigate you through various advanced techniques and considerations about memoization that can greatly enhance your repertoire, especially when tackling large-scale applications.

Our interactive approach won't just equip you with the knowledge to use memoization but will also engage you in actively exploring how to handle potential future scenarios. Delve into this guide to unlock the power of memoization for your JavaScript applications, make them run faster and smoother without necessarily consuming more memory. Whether you're optimizing a complex piece of software or just eager to learn new JavaScript strategies, this guide has something to peak your interest.

Grasping the Concept of Memoization in JavaScript

In JavaScript, memoization is a technique focused on enhancing performance by preserving the results of expensive function calls. These results are retained in a cache, a structured data storage that holds copies of data that allows faster access. Upon repetition of identical inputs, previously stored results are re-accessed, eliminating the need to recalculate. Memoization serves as the cache system of functional programming, predominantly applicable for functions iterating over identical inputs.

To truly appreciate memoization, one must first comprehend JavaScript's concept of closures. A closure is created when a function retains access to its scope even after execution. This data retention capability in an isolated scope facilitates memoization, acting as a handcrafted cache container. Bulky computations are transferred into a space primed and ready to deliver results without iterating through the calculations again.

Take, for instance, a computation-intensive function. Upon the first encounter with a unique input, the function processes the series of calculations, persisting the output in the cache within the scope. This stored output is indexed with the input as the reference key. When the same input recurs, calculation repetition is unnecessary; the cache retrieves the previously stored result via the input key.

const memoize = (fn) => {
    let cache = {};
    return (...args) => {
        let n = args[0]; // Considering single argument here.
        if (n in cache) {
            return cache[n]; // Return stored result
        else {
            let result = fn(n);
            cache[n] = result; // Store the result in cache
            return result;
const fib = memoize(
    (n) => n < 2 ? n : fib(n - 1) + fib(n - 2)

Let's look at the Fibonacci sequence as an example. For those unfamiliar, the Fibonacci sequence is a series of numbers in which each number is the sum of the two preceding ones, generally starting with 0 and 1. The original JavaScript algorithm without memoization would handle higher input values inefficiently due to repeated calculations. However, implementing the memoization technique allows retrieval of repeated Fibonacci numbers from the stored memory, thus significantly improving retrieval speed.

But when is the pivotal moment to apply memoization wisely? You should consider memoization when:

  1. Dealing with heavy, frequently recurring identical calculations.
  2. Accessible memory is abundant.

In scenarios involving simpler computations or less frequent identical inputs, ramping up memory usage for memoization might not always be optimum. There is a trade-off between time complexity, operating speed, and space complexity or memory usage. Hence, applying memoization requires a considered approach, taking into account the computational load, frequency of identical inputs, and available memory.

By now, a crucial question might be simmering in your mind, "In what scenarios can you foresee memoization being a game-changer for your applications?"

Evaluating the Implications of Memoization in JavaScript

The most widely acknowledged benefit of employing memoization in JavaScript is enhanced function execution speeds as complementary to pure function calls. Suppose we consider executing a function as a computational task. In that case, memoization essentially reduces that task's runtime complexity from O(n) to O(1), where n is the number of function parameters. As a result, the effectiveness of memoization increases exponentially with repetitive calls to the same function using the same parameters. Notably, computational tasks that involve recursive operations or looping benefit the most, given the recurring nature of their execution paths.

The seemingly perfect performance gain does not come without a cost: memos. These are essentially memory slots that store previously executed function outcomes for later use. Thus, the more memos you have for a function, the more memory it will consume. For instance, a factorial calculator that employs memoization will store all previously calculated factorials, causing a linear increase in memory footprint. This dictates that e_app developers carefully evaluate the complexity-memory trade-off inherent in the function they are attempting to optimize.

To visualize this trade-off, consider JavaScript's popular recursive function to calculate Fibonacci numbers. Without memoization, the function exhibits an exponential runtime behavior due to the repetitive computation of previous Fibonacci numbers. However, incorporating a memo implementation can reduce this runtime substantially at the expense of memory utilization.

function fib(n, memo = {}) {
    // Memo check
    if (memo[n]) return memo[n];
    // Base case
    if (n <= 2) return 1;
    // Recursive case with memoization
    memo[n] = fib(n-1, memo) + fib(n-2, memo);
    return memo[n];

That said, memoization should not become a quick fix for every function in your codebase. Indeed, some function calls may not leverage memoization benefits due to their specific usage patterns or their natures. For instance, a function that always receives unique parameters will not benefit from memoization as it will never hit a cache. Therefore, careful application profiling is required to determine whether memoization is an appropriate optimization strategy.

All in all, memoization is a potent mechanism for improving runtime performance in functional JavaScript. However, these enhancements come at the expense of increased memory utilization. Finally, while memoization can be beneficial in the right circumstances, it should be applied deliberately and selectively after thorough analysis and profiling.

Mastering the Implementation of Memoization

Let's start with the basic implementation of memoization in a single-parameter function. Consider a function calculateFactorial(), which calculates and returns factorial of a number. The memoized version would look something like this:

let memo = {};
function calculateFactorial(num) {
    if(memo[num]) return memo[num];
    else if(num == 0) return memo[num] = 1;
    else return memo[num] = num * calculateFactorial(num - 1);

The above code first checks if the result is already present in memo. If it's there, the function immediately returns the result, thus saving computation time. This is the crux of memoization; storing the results of expensive function calls and reusing them when the same inputs occur again.

Both readability and performance can be enhanced by encapsulating the caching logic in an external function. Let's wrap memoization into a higher-order function:

function memoize(fn) {
    let cache = {};
    return (arg) => {
        if(arg in cache) return cache[arg];
        else return cache[arg] = fn(arg);

This memoize() function accepts a function fn as a parameter and returns a new function that takes a parameter arg. This new function checks if the result is in the cache and returns it if present. Otherwise, it calculates the result, stores it in cache, and then returns it.

Dealing with multi-parameter functions can get a bit trickier. To memoize a function calculateSum(a, b), a key needs to be generated for every unique set of arguments. A quick way to do this is by converting arguments into a string using JSON.stringify() method. Here is how the modified memoize() function would look:

function memoize(fn) {
    let cache = {};
    return (...args) => {
        let key = JSON.stringify(args);
        if(key in cache) return cache[key];
        else return cache[key] = fn(...args);

By mastering the implementation of memoization, significant performance increase can be achieved in JavaScript applications. Be mindful of memory usage though, as storing extensive amounts of intermediate results could lead to memory overflow. It's a trade-off between speed and memory that programmers need to delicately handle. Given different situations, consider: is the decreased time complexity worth the increased space complexity?

Debunking Common Memoization Mistakes

Debunking Common Memoization Mistakes

Function memoization in JavaScript has optimal utility in scenarios that involve computationally intensive, deterministic calls. Misuse of memoization on inadequate functions invites issues.

Case One: Non-Deterministic Functions

A non-deterministic function like the generation of random numbers is not memoization-friendly. As this function type doesn't produce consistent outputs for the same inputs, the application of memoization offsets the expected result. Here's an illustrative example:

// The function generates a random number
function getRandomNumber() {
    return Math.random();

// This is a generic illustration of a memoization function. Note that JavaScript doesn't have a built-in method to memoize functions.
function memoize(fn) {
    let cache = {};
    return (...args) => {
        let n = args[0]; 
        if (n in cache) {
            return cache[n];
        else {
            let result = fn(n);
            cache[n] = result;
            return result; 

// Attempt to memoize getRandomNumber
let memoRandomNumber = memoize(getRandomNumber);
memoRandomNumber();  // The outcome is unexpectably a random result
memoRandomNumber();  // It doesn't return a fresh random number as desired

Listen carefully! The memoRandomNumber function is likely to return the same 'random' number each time. This doesn't align with the unpredictability we expect from a random number generator. Stick to deterministic functions when applying memoization. Can you identify other non-deterministic functions which have adverse effects when memoized?

Case Two: Memory Management

Balancing memoization with efficient memory management, such as cache eviction for enduring execution cycles, prevents memory bloating. Regrettably, the following Fibonacci function fails to consider this.

// This Fibonacci function utilizes a cache
let fib = (n, cache = {}) => {
    // A successful cache hit is indicated by this comment
    if (cache[n]) return cache[n];  

    // Establishing base cases
    if (n <= 1) return n;  

    // The large Fibonacci numbers trigger cache updates
    return cache[n] = fib(n - 1, cache) + fib(n - 2, cache);  

In constant use, this Fibonacci function can provoke memory bloating since it lacks efficient cache eviction mechanisms. Reflect on this: How would you implement cache eviction in this Fibonacci function to prevent potential memory issues?

Case Three: Internal State Changes

Memoization's pivotal role is sidestepping complex computations rather than catering to internal state transitions. Functions that mutate or rely on external states often deliver surprises when memoized. Look at the following illustration:

// Declare an external variable
let count = 0;

// Define a function that depends on external state
const countPlusOne = () => count++;

// Memoize the function
const memoCountPlusOne = memoize(countPlusOne);
memoCountPlusOne();  // returns 0
memoCountPlusOne();  // it still returns 0, rather than the anticipated 1

This example stresses that memoization of functions with mutable external state dependencies may land you in unexpected consequences. Can you think of the possible erratic behaviours you might encounter if you memoized a function that modifies an external database?

Case Four: 'this' Context

Preserving the 'this' context within functions during the memoization process is a challenge worth our attention. The preservation of the correct 'this' context profoundly impacts the expected conduct of the function, particularly within object methods.

// An object named 'user' with a greet method
const user = {
    name: 'Alice',
    greet: function() {
        return `Hello, ${}!`;

// Memoize the 'greet' method of 'user'
user.greet = memoize(user.greet);

Memoizing 'greet' severs its bond with 'this', yielding an undesired output. Handle with care when memoizing object methods relying on 'this' or consider binding the method to its context. Can there be instances where jeopardizing the 'this' context can be beneficial?

To conclude, let's remember to always harmonize memoization with careful function behaviour considerations, attentive resource management, and a comprehension of JavaScript's subtleties. This pragmatism will accelerate your journey in harnessing memoization's full potential within functional JavaScript.

Advanced Techniques and Considerations for Memoization

Firstly, when dealing with large-scale applications, understanding the memory usage of memoization is extremely important. Remember that memoization involves storing the results of computations, which can quickly consume a large amount of memory in more substantial applications. Therefore, implementing a mechanism that caps the size of your cache can prevent excessive memory usage. Let's look at a quick example of such a cache:

function memoizeWithLimit(fn, limit) {
    let cache = new Map();
    return function() {
        let key = JSON.stringify(arguments);
        if (cache.has(key)) {
            return cache.get(key);
        let result = fn.apply(undefined, arguments);
        if(cache.size >= limit) {
            // Deletes the oldest element in the cache
            const firstKey = cache.keys().next().value;
        cache.set(key, result);
        return result;

Here, we use a JavaScript Map to implement the cache. Maps maintain the insertion order of the elements, allowing us to identify the oldest items in the cache when the limit is reached and purge them.

Next, effectively recording dependencies can ensure that your cached values are invalidated when necessary dependencies change. This is particularly important in more complex applications where it is relatively common to see changes in one area of your program invalidate computations cached in another.

Another common consideration when using memoization pertains to the use of pure functions. When using memoization with functions that are subject to side effects or depend on mutable external state, it can lead to unpredictable and inaccurate results.

It's also important to remember that memoization might not always be the most efficient approach to improving your program's performance. Given the additional memory usage, the overhead of caching computations, and the complexity it adds to your code, you might find that other strategies, such as optimizing your algorithms or improving your data structures, will yield better results.

Finally, let's ponder these questions. How would you approach situations where your function’s outputs are less deterministic or depend on resources or data outside of its scope? How would you handle concurrently running identical functions to ensure memoization still works effectively? Keep thinking about how to measure the real-world effectiveness of memoization, and how you can keep track of your cache's performance in real-time. This way, you can make clear and informed decisions about whether or not memoization is providing the performance boost you're after.


In this comprehensive guide on memoization in JavaScript, the author explores the concept, implementation, and implications of using memoization for performance optimization in functional programming. The article begins by explaining how memoization works by storing the results of expensive function calls in a cache, and how it can significantly improve the speed and efficiency of repetitive computations. The author provides step-by-step guides on implementing memoization in code, highlighting common mistakes to avoid. They also discuss advanced techniques and considerations for memoization, such as managing memory usage, handling dependencies, and the importance of using pure functions.

Key takeaways from the article include: the benefits of using memoization in computational tasks, the trade-off between time complexity and memory usage, the need for careful application profiling to determine when memoization is appropriate, and the challenges of memoizing non-deterministic functions, managing memory, handling internal state changes, and preserving the 'this' context.

To challenge the reader, an interesting task would be to implement a cache eviction mechanism in a memoized Fibonacci function to prevent memory bloating. The task requires thinking about how to efficiently clear out unnecessary cache entries as new computations are made, ensuring that only the most recent results are stored in memory. This task provides an opportunity for the reader to apply their understanding of memoization and memory management in a practical context.

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