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3 useful built-in objects and functions in JavaScript

Phineas Jensen

By Phineas Jensen
June 15, 2022

Three semi trucks parked on concrete, with a dog standing watch over them

I love learning (and learning about) programming languages. Right now I’m teaching myself Rust and trying to learn about 3D rendering. Every time I find a list of programming languages I have to look up every one that I haven’t heard of, and when I see posts about Zig or Haskell or any other cool project on Hacker News, I can’t help reading through the comments and seeing people discuss the unique features, quirks, and drawbacks of each language.

One thing I enjoy about learning about these languages (sometimes in an all-too-shallow way) is seeing all the different methods and syntaxes that exist for solving problems, and while it’s always tempting to think the grass is greener on the other side, it’s also important to do the same kind of exploration within the languages I’m using right now. Not only is it important, it actually makes using those languages a lot more enjoyable as I find new, more efficient ways to do things I’ve probably done dozens of times before.

With that spirit, here’s a little list of cool objects and functions in JavaScript that have given me that feeling of excitement and made the language more fun and satisfying to use. Like any JavaScript feature, support will vary from browser to browser, runtime to runtime, and version to version, but with tools like Webpack becoming ubiquitous, that’s becoming less of a problem.

The Set object

The Set object has a lot of use cases, but one I find myself using a lot is a set of selected items in a list like this:

A table of items which can be selected with check boxes

Why is Set a good fit for this? Because a Set is a list of unique items (we shouldn’t have one item in the selected list twice), we can add and remove items from it easily, and we can check membership with a single, obvious method that returns a boolean. Let’s look at an example using an array first, which mimics code I’ve seen quite a bit.

const selected = [];

function select(item) {
    if (selected.indexOf(item) == -1) {

function deselect(item) {
    const index = selected.indexOf(item);
    if (index != -1) {
        selected.splice(index, 1);

function isSelected(item) {
    return selected.indexOf(item) != -1;

This isn’t too complex, and it works well, but it’s pretty verbose. Using the indexOf function three times is a little messy, and the splice function is uncommon enough that it’s easy to forget exactly how it works. We can implement the same functions more simply using a Set:

const selected = new Set();

function select(item) {

function deselect(item) {

function isSelected(item) {
    return selected.has(item);

In fact, I’d say this functionality is so much simpler that we don’t even need to define new functions. Selecting an item by calling selected.add(...) reads just as well as or better than select(...), and the same goes for these other function names. Set handles it all for us.

Note that if you use a Set in React with useState, the component won’t re-render unless you create a new Set object when you update the state, like so:

function Example(props) {
    const [selected, setSelected] = useState();
    const addItem = (item) => {
        setSelected(new Set(selected));

    return items.map(item => (
        <button onClick={() => addItem(item)}>

This is because React will bail out of the state update if the values are the same according to the Object.is function.

The URL API’s URLSearchParams object

Imagine you’re writing a function that needs to parse and use a URL’s search parameters. Pretty simple task, right? It wouldn’t be too hard to do by hand, but there’s probably a library out there somewhere, so you do a Google search for “javascript parse url parameters” and click on the first Stack Overflow answer in the results. You’re greeted with an answer that defines a function using .split() a few times and then a much longer and more complex function that decodes the parameters in a non-standard way. Is this really necessary?

With the built-in URLSearchParams object, no. Supported in every major browser except Internet Explorer (which just reached its end of support life), this nice object handles parsing and formatting of string parameters with proper encoding and decoding:

const params = new URLSearchParams('?foo=1');

params.append('bar', 'has weird & tricky characters'); // Add another parameter
console.log(params.toString()); // Prints 'foo=1&bar=has+weird+%26+tricky+characters'

It handles multiple parameters with the same key easily and supports iteration:

const params = new URLSearchParams('?foo=1&foo=2&foo=3');
console.log(params.getAll('foo')) // Prints ["1", "2", "3"]

for (const [k, v] of params) {
    console.log(`${k}: ${v}`);
// Output: 
// foo: 1
// foo: 2
// foo: 3

Much nicer, and requires no copy-pasting of code from Stack Overflow or installing dependencies!

The Array iterator functions

If you’ve used a lot of JavaScript or used languages like Perl, Python, Ruby, Rust, functional languages, or languages with iterators, you’ve probably seen functions like map or forEach. I’ve used them pretty extensively but still find that a lot of people don’t seem to know about them. For people coming from languages like C or C++, where they aren’t available, or for people who are fresh out of a university program where neat things like that are often not taught in favor of theory, that’s not much of a surprise.

Array.prototype.forEach is pretty straightforward. Compare the following equivalent code snippets:

const names = ["bob", "roberta", "alice", "reza"];

// This...
for (let i = 0; i < names.length; i++) {
    console.log(names[i].substr(0,1).toUpperCase() + names[i].substr(1));

// ...is equivalent to this:
names.forEach(value => console.log(value.substr(0,1).toUpperCase() + value.substr(1)));

forEach takes as its argument a callback function, which it then calls once for each item in the array. The callback function can take more arguments than just the value as well; see its MDN page.

I personally find map more interesting and useful. Map is very similar to forEach, except that the values returned by the callback function are assembled to make a new array of the same length as the old one, essentially returning a transformed array.

const names = ["bob", "roberta", "alice", "reza"];

// This...
const uppercaseNames = [];
for (let i = 0; i < names.length; i++) {
    const name = names[i];
    uppercaseNames.push(name.substr(0,1).toUpperCase() + name.substr(1));

// ...is equivalent to this:
const uppercaseNames = names.map(name => name.substr(0,1).toUpperCase() + name.substr(1));

There are similar cool functions that return true or false if the callback function returns true for every or some item in the list:

const names = ["bob", "roberta", "alice", "reza", "spaces are good"];
// Both values will be false because of the "spaces are good" item
const allSpaceless = names.every(name => name.indexOf(" ") === -1);
// or...
const allSpaceless = !names.some(name => name.indexOf(" ") !== -1);

Or we can find individual or multiple items using a function, rather than just by value:

const names = ["bob", "roberta", "alice", "Reza", "spaces are good"];
const namesStartingWithR = names.filter(name => name[0].toLowerCase() === "r") // returns ["roberta", "Reza"]
const firstNameWithC = names.find(name => name.toLowerCase().includes("c")) // returns "alice"

Then there’s the complex but very cool reduce function, which starts with an initial value and replaces it with the result of the callback function on each iteration. map can be implemented using reduce:

const names = ["bob", "roberta", "alice", "reza"];
// This...
const uppercaseNames = names.map(name => name.substr(0,1).toUpperCase() + name.substr(1));
// ...is equivalent to this:
const uppercaseNames = names.reduce(
    (previousValue, name) => {
        previousValue.push(name.substr(0,1).toUpperCase() + name.substr(1));
        return previousValue;
    [] // Initial value

Obviously, it’s not terribly useful in this case, but can be extremely useful for things like summing items in a list or transforming a list into an object somehow.

Consider keeping the Array reference on hand to keep these functions close to your heart and start to learn some of their other features. As new versions of ECMAScript come out, new prototype functions will crop up too. Note that some of these functions will vary in performance compared to normal loop iterations. If you’re using them in heavy workload or low-latency scenarios, make sure to compare and benchmark performance as needed.


Okay, that was actually more than 3 things, but they fit into three categories, so I’m keeping the post’s name. JavaScript’s built-in objects and the generally available Web APIs have a lot more cool APIs and functions and features that are very much worth checking out. MDN (which I linked several times throughout this article) is a fantastic resource to learn about them, so make sure to get familiar with it as you work with JavaScript. It’ll pay off.

javascript tips