Array methods, and when to use forEach, map, and reduce

Written by Published: 🍿 6 min. read

First thing to get out of the way is that each of these methods returns something different.

.forEach only loops through the array it is called on. This means that it returns undefined.

For .map, it returns a new array where each item is the result of the given callback.

The much more complex method, .reduce, can return anything. The return type is determined by the second argument passed and the callback.

Here are some examples:

const iceCreamTypes = [
    'vanilla',
    'chocolate',
    'strawberry',
    'mint'
];

const forEachResult = iceCreamTypes.forEach((type, index) => {
    // Capitalize the icecream names
    iceCreamTypes[index] = type[0].toUpperCase() + type.slice(1);
})

console.log(forEachResult) // undefined

// Add organic types
const newIceCreamTypes = donutPrices.map((type, index) => {
    return type + ' Organic';
});


// Merge the two arrays together in blissful frozen harmony
const finalIceCreamTypes = iceCreamTypes.reduce((allIceCreamTypes, currentIceCreamType, index) => {
    allIceCreamTypes.push(currentIceCreamType);
    allIceCreamTypes.push(newIceCreamTypes[index]);

    return allIceCreamTypes;
}, []);

Why use .forEach instead of for in or for of

Personally, it depends on preference. Call me old fashioned, but I like using for loops. If I need to do some stuff to every item in an array, I'll usually reach for for loops. If I am writing that same for loop everywhere, then it is time to put it in a callback .forEach.

Why use .map instead of for in or for of

Let's say you need to run a callback on each item in an array, but need a new array with the results of each callback. With for of then you would likely end up with something like this:

const itemArr = [...];
const updatedItemArr = [];

for (const item of itemArr) {
    // run some code to alter the item
    updatedItemArr.push(item);
}

But with .map, that example looks more like this:

const itemArr = [...];
const updatedItemArr = itemArr.map((item) => /*run some code to alter the item */);

It is more explicit to me to do it the second way instead of the first.

Why use .reduce instead of .forEach, .map, for in, for of

The .reduce method is a powerhouse if you know what it can do. For example, I need to make a menu string based on an object that is returned from a database.

const returnedMenuData = [
    {
        name: 'pasta',
        price: 16,
    },
    {
        name: 'burger',
        price: 12,
    },
    {
        name: 'pizza',
        price: 14,
    }
];

const formattedMenuTemplate = returnedMenuData.reduce((templateString, { name, price }) => {
    const newName = name[0].toUpperCase() + name.slice(1);
    const formattedPrice = currencyFormatter.format(price); // Assume this is an Intl.NumberFormat object

    const template = `
        <div class="menu-row">
            <p>${newName}</p>
            <hr />
            <p><small>${formattedPrice}</small></p>
        </div>`

    return templateString + template;
}, '');


MenuElement.insertAdjacentHTML('beforeend', formattedMenuTemplate);

You can see we were able to update each value, convert it into a template, and return the result as a string to be used as a template for our menu.

When you reduce an array, it is easier to perform multiple array methods at once.

const returnedMenuData = [
    {
        name: 'pasta',
        price: 16,
        onSale: false,
    },
    {
        name: 'burger',
        price: 12,
        onSale: true,
    },
    {
        name: 'pizza',
        price: 14,
        onSale: true,
    }
];

const onlyItemsOnSale = returnedMenuData.reduce((templateString, { name, price, onSale }) => {
    if (!onSale) return templateString;

    const newName = name[0].toUpperCase() + name.slice(1);
    const formattedPrice = currencyFormatter.format(price); // Assume this is an Intl.NumberFormat object

    const template = `<div class="menu-row">
        <p>${newName} <b>ON SALE NOW!!</b></p>
        <hr />
        <p>${formattedPrice}</p>
    </div>`
}, '');

OnlyTheSpecialsMenu.insertAdjacentHTML('beforeend', onlyItemsOnSale);

In this example, we've done .filter and .map in a single callback.

That in mind, .reduce should not be the first tool in your tool belt.

In summary...

Each method has their own purpose, but knowing when is tough.

Do you have a simple array that you need to loop through? You can use .forEach.

Do you need to get a new array from the results of your callback? You should use .map.

Do you have a complex series of tasks, such as filtering, flattening, and mapping, and doing it all in one function would make life easier? Then it's time you learn .reduce.