What is memoization in JavaScript?
Tags: javascript
Published: 2021-12-05 22:01:17

**Memoization** is a specific form of caching used in dynamic programming. **Caching** is a way to speed our programs and hold some data in an accessible box for later use. It stores the pre-computed value and returns the value instantaneously if the given input is seen before. 

In addition, memoization is an optimization technique in caching results when the same set of arguments result in the same output and lead to performant web applications.
  
## Fibonacci sequence

Let's write a function to compute the Fibonacci sequence without memoization.

> The **Fibonacci sequence** represents a list of numbers where each value is the sum of the two previous values. The first two elements of the sequence are 0 and 1.

```js
// fibonacci without memoization
const fib = (num) => {
    if (num < 2) {
        return 1
    } else if (!num || typeof num !== 'number') {
        return 'value must be a number!'
    }
    return fib(num - 1) + fib(num - 2)
}

console.log(fib(10))
```
From the code above, we have a function that generates the sum of two previous values. Since the function is pure, `num` as the given value of the argument in `fib()` will return the same value. Therefore, we have a recursive function in this code sample in the `return` condition. Suppose `fib()` caches the results. In that case, as we will see later on, the program's performance could have given a quicker response because we could have stored the previous result of the sequence instead of running the calculation all over.  

## Memoizing the fibonacci function
In the example below, we will see how the `fib()` function memoizes its given input.

```js
// fibonacci sequence with memoization to run the function fib()
let cache = {}; // set cache
const fib = (num) => {
    // if exists in cache return from cache
    if (cache[num] !== undefined) {
        console.log(`${num} is cached!`);
        return cache[num];
    }
    // if not in cache perform operation
    cache[num] = num < 2 ? 1 : fib(num - 1) + fib(num - 2);
    return cache[num];
}
const result = fib(5)
console.log(result) // 8
```
In the code snippet above, we created a cache object that the `fib()` uses to store its output value. Each time `fib()` is invoked, it checks whether the `fib()` of input `num` has been stored previously in the cache object. If it has, it immediately returns the cached value.

Now computing `fib(5)` after `fib(10)` will significantly speed up the program's performance as fib(5) in the function is calculated as part of fib(10), which leads to efficient code. 

> The time it takes when caching is halved once the function recognizes a given input declared in the program before.
  
## Exercise
Let's find the factorial of a given number using the cache execution of a `factorial()` function.

```js
// factorial of a number with memoization (cache execution)
let cache = {}; // set cache
const factorial = (num) => {
  // if cache already exists, return cache
  if (cache[num] !== undefined) {
    console.log(`${num} is cached!`);
    return cache[num];
    // edge case validation for not a number
  } else if(!num || typeof num != 'number') {
      return `value must be a number`
    }
  cache[num] = 
    num === 0 
    ? 1 : num === 2 ? 2 
    : num * factorial(num - 1); // condition ternary operator, same with if/else statement
  return cache[num];
};

console.log(factorial(5)); // 120
```