Day 10 of 21 Days of Javascript

Michael Tefula

Michael Tefula

— 1 min read

Today I covered more higher order functions. Map and reduce. Yesterday I covered filter. One of the benefits of using these functions is composability. This is the idea of using relatively small blocks of code chained together for more complex behaviour.

Here's a quick example. If we have a list of numbers [1, 2, 3, 4] and we want to transform each number by firstly, adding 2 to it. We then want to filter the list to just odd numbers. Here's how you might write that with just one block of code:

let myList = [1, 2, 3, 4];

const result = [];
let new_num;

for (let num of myList) {
    new_num = num + 2;           // add 2 to each number
    if (new_num % 2 !== 0) {     // check if it’s odd
        result.push(new_num);    // add to results list if its odd
    }
}

console.log(result); // will print [3, 5]

That code works fine. But we could use Javascript's built-in higher order functions and do the following with just one line:

let result = myList.map(n => n + 2).filter(n => n % 2 !== 0)

Here we are using composability. That is, we have two higher-order functions (map and filter) working on our inputs. See the full code example below.


let myList = [1, 2, 3, 4];

const result = [];
let result = myList.map(n => n + 2).filter(n => n % 2 !== 0);
console.log(result); // will print [3, 5]

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