Javascript on Jake Worth

JavaScript Reflection with toString

Fri, 10 Jul 2026 12:04:19 -0400

Reflection is an important tool in programming; it lets us examine a language’s structure. Can we reflect in JavaScript? Sort of.

Some functions can be reflected with toString():

> const one = () => 1
undefined
> one.toString()
'() => 1'

Very cool! But native functions only return this:

> Math.max.toString()
'function max() { [native code] }'

You must read the ECMA Script specification to know what the function is required to do. The actual implementation is written in the JavaScript engine, likely in another language like C++, which is why Node can’t show it via toString().

Focus a JS Test

Fri, 10 Jul 2026 11:48:47 -0400

A vital testing skill is focusing a test. When iterating on a failing test, we want that test under microscope.

The way I do this in JavaScript is .only:

import {describe, expect, test} from 'vitest';
import {sum} from './sum.js';

test('adds 1 + 2 to equal 3', () => {
 expect(sum(1, 2)).toBe(3);
});

describe.only('adding fours', () => {
 test('adds 4 + 4 to equal 8', () => {
 expect(sum(4, 4)).toBe(8);
 });
});

JSDoc Deprecations in TypeScript

Fri, 10 Jul 2026 10:35:05 -0400

JSDoc has a @deprecated tag that you can use in TypeScript, too. Here’s an example.

NPM List Packages

Tue, 07 Jul 2026 10:00:34 -0400

When upgrading a dependency for a security issue, we might upgrade and move on:

npm install <dependency>@<version>

But wait! Was every <dependency> version upgraded, or just the version in package.json? Check with:

npm list // or npm ls

Node REPL, No Preview

Tue, 02 Jun 2026 15:46:17 -0400

After version v12.17.0, Node’s REPL provides a useful preview effect. In this example, 2 is shown in gray before I hit enter.

> 1 + 1
2

But what if I don’t want that? Here’s a file called repl that disables it as an option:

#!/usr/bin/env node

const repl = require('node:repl');
repl.start({preview: false});

Then:

$ ./repl
> 1 + 1

No preview.

Docs

NVM Install and Use Latest LTS

Fri, 29 May 2026 13:49:11 -0400

With my Node versions managed via NVM, I want to be on the latest LTS. You can ensure that happens with these commands:

$ nvm install --lts
$ nvm alias default lts/*
default -> node (-> v24.16.0)

JavaScript Wraparound Index

Thu, 07 May 2026 09:44:10 -0400

What I call “wraparound” are repeatable indices on an array. The common use case is a carousel. Here’s how it’s done:

const items = ["first", "middle", "last"];
const i = 0; // Our iterable index

// "Next" ->
const nextIndex = (i + 1) % items.length;

// <- "Previous"
const prevIndex = (i - 1 + items.length) % items.length;

JavaScript String Concatenation

Wed, 06 May 2026 10:39:46 -0400

Here’s a periodic reminder that JavaScript supports string concatenation with a plus sign:

> 'foo' + 'bar'
'foobar'

JavaScript Spread a String Into An Array

Tue, 05 May 2026 10:49:25 -0400

Need an array of letters? Here’s a trick:

> const alpha = [...'abcdefghijklmnopqrstuvwxyz']

JavaScript Array Join Comma Default Separator

Tue, 05 May 2026 09:48:19 -0400

When you join an array in JavaScript, the default separator is a comma:

node> ['j', 'a', 'k', 'e'].join()
'j,a,k,e'

JavaScript Bitwise AND

Fri, 01 May 2026 14:53:41 -0400

The JavaScript bitwise AND operator (&) can be used for some real-world tasks, like testing if a number is odd or even.

Initialize With npm init

Thu, 12 Feb 2026 11:19:46 -0500

Most JavaScript projects come with a package.json. But what if you want to make a new one?

Writing the DOM with fs

Fri, 30 Jan 2026 11:12:52 -0500

How can I output the entire DOM to a file in a JavaScript test?

Rebuilds on Write With Nodemon

Tue, 30 Dec 2025 16:42:28 -0500

I’ve been hacking on a TypeScript file all day, but I need transpiled JavaScript for any testing. How did I reduce cycle time (and mitigate many chances to forget to build at all)?

npmrc "engine-strict"

Tue, 16 Dec 2025 16:38:36 -0500

Suppose your package.json specifies a Node engine of greater than or equal 24. Does this mean all engineers on your team will use Node 24? No, unless you use this one weird trick!

Learning JavaScript Promises the Feynman Way

Mon, 25 Aug 2025 12:08:06 -0400

Want to learn a tricky topic and sharpen your learning skills at the same time? In this post, I’ll use the Feynman Learning Technique— a method of learning complex things by explaining them simply— with a sprinkle of LLM magic, to deepen my understanding of JavaScript promises.

Editing in Nodes REPL Editor

Tue, 22 Jul 2025 16:32:43 -0400

The Node REPL has its own editor:

node
> .editor
// Entering editor mode (Ctrl+D to finish, Ctrl+C to cancel)
const increment = (array) => array.map(item => item + 1)
increment([1,2,3])

Exit and evaluate with Ctrl+D:

[ 2, 3, 4 ]

I’d use this to experiment with JavaScript API’s without leaving my terminal-based IDE.

Why I'm "All In" on TypeScript

Wed, 26 Mar 2025 10:47:45 -0400

I’ve added TypeScript to several projects I’ve worked on. In this post, I’d like to discuss why I think TypeScript is essential and document my expectations around it.

Load a Dependency in the Node REPL

Thu, 11 Jul 2024 10:42:11 -0400

Exploring your JS dependencies locally is a great way to learn and experiment. Here’s how to load a dependency from your /node_modules directory into the Node REPL.

$ node
> cn = require('classnames')
> cn("always", { never: false, sometimes: true })
'always sometimes'

Absolute Imports in TypeScript and React: A Practical Guide

Tue, 02 May 2023 00:00:00 +0000

Absolute imports are an essential developer experience feature for me in any JavaScript application. In this post, I’ll explain what they are, how to use them, and why they matter.

Sorting Numbers with JavaScript's sort Function

Fri, 10 Feb 2023 10:04:55 -0500

Today in “Let JavaScript Surprise You”:

let array = [1, 20, 11, 10, 7, 17, 2];
console.log(array.sort());

> [1, 10, 11, 17, 2, 20, 7]

😳 The numbers are not sorted as we might expect. What’s going on here? From Mozilla’s docs:

The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.

Now this sorting makes sense! Luckily, sort accepts a function, so here’s the sorting we intended:

const sortNumbers = (first, second) => first - second
console.log(array.sort(sortNumbers))

> [1, 2, 7, 10, 11, 17, 20]

How to Create a Timer or Polling in React with setInterval

Thu, 29 Dec 2022 00:00:00 +0000

Have you ever wanted to create a timer in a React app? This could be in support of a UI timer or polling. In this post, I’ll explain how to create a timer effect in a React application using hooks and setInterval.

Build a Sentence from a JavaScript Array

Thu, 22 Dec 2022 09:12:08 -0500

Have you ever wanted to take a JavaScript array of strings and turn them into a sentence? Here’s how you do that.

const toLearn = [
 'object-oriented programming',
 'frontend engineering',
 'testing',
]

const sentence = new Intl.ListFormat().format(toLearn)
> "object-oriented programming, frontend engineering, and testing"

ListFormat takes options.

This has cross-browser support! The days of building a ’to sentence’ function in JavaScript are over.

Logging an Object in JavaScript

Thu, 07 Jul 2022 00:00:00 +0000

When printing a JavaScript value to the console, I suggest using an object literal over the raw value.

Only One Way Out of a Function

Mon, 02 May 2022 00:00:00 +0000

A programming style I try to practice could be described as: “there should be only one way out of a function.” Early returns can often cause more confusion than they’re worth. When possible, I avoid them in favor of a single return.

How to Organize JavaScript Imports

Mon, 30 Aug 2021 00:00:00 +0000

The import statements at the top of a JavaScript component file can be a confusing, duplicative, churning mess. Is there a way to organize them that makes sense and scales? In this post, I’d like to share the way I handle this detail.

How I Clean Up JavaScript Dependencies

Mon, 03 Aug 2020 00:00:00 +0000

Unused dependencies are bad: they increase the size of your project, slow down your processes, require upgrades, and send incorrect messages to fellow developers about what’s important. Make your project better by periodically auditing your dependencies, and removing those that are unused.

JavaScript Equality

Tue, 17 Jul 2018 00:00:00 +0000

A few weeks ago, I built an app with React.js and create-react-app that I call ‘JavaScript Equality’. View deployment here. This application demonstrates the JavaScript value-comparison operators == and ===. It’s inspired by the JavaScript Equality Table.

Build URLs with Window Location

Wed, 23 Sep 2015 21:45:56 -0500

Want to build URLs with JavaScript? Try thewindow.location object.

This came from a React.js project we’re working on. It uses ES6 template string interpolation, which isn’t necessary but definitely is nice:

`${window.location.protocol}//${window.location.host}/posts/${this.props.postID}/edit`

Which becomes:

"https://secret-project.com/posts/42/edit"