TL;DR: Use ts-prune is in maintenance mode. Use knip to find dead code instead. It's great!

If a variable gets a type but no one looks at it, does it really get a type at all? This post looks at how type inference is implemented in the TypeScript compiler. It's of some interest to anyone...

Over the past few months I became a TypeScript contributor and implemented a new feature, type predicate inference, that should be one of the headliners for TypeScript 5.5. This post tells the story...

I've written many words about TypeScript and I'm sure I've read even more. Here are four words that make me cringe every time I see them. If you write about TypeScript, please steer clear of these!

It's Christmastime and I've been happily working through this year's Advent of Code in Deno (look forward to a blog post in the new year). What with all the presents, it's a good time to think about...

Learn the key differences between interfaces and type aliases in TypeScript, including their use cases and important features to consider.

Hang out on the internet much and you'll hear gripes about how TypeScript isn't "sound," and that this makes it a poor choice of language. In this post, I'll explain what this means and walk through...

I started the Effective TypeScript blog on March 6, 2020. It was my fourth day staying home from work with a lingering illness. I had no idea that ten months later, I'd still be working remotely....

Iterating over the keys and values in an object is a common operation that's surprisingly hard to write without type assertions in TypeScript. This item explains why the types you get from Object.k...

The New TypeScript Handbook has some real gems in it. Here's what it has to say about generics: Writing generic functions is fun, and it can be easy to get carried away with type parameters. Having...

Bug Report This was the one type error I got when I updated my project from TS 4.1 → 4.2. It's a false positive so I figured I'd file an issue in case others run into it as well. I assume t...

Please verify that: There is an associated issue in the Backlog milestone (required) Code is up-to-date with the main branch You've successfully run hereby runtests locally There are new o...

This post looks at how TypeScript's type union operator (| aka "pipe" or "or") is really an inclusive or, and how this can come as a surprise. It presents a few options for getting an exclusive or if...

Readers of Effective TypeScript and followers of this blog will know that testing types is a long-standing interest of mine. In this post I show how eslint-plugin-expect-type provides new syntax and...

Does something look odd about this unit test?

Type guards are a powerful tool for improving TypeScript's built-in control flow analysis. This post looks at when it's appropriate to use a type predicate, and in particular what it means when a...

With this PR we optimize control flow graph construction to omit conditional constructs that have no control flow effects. Consider this example: function test(x: A | B | C) { let str = x instanc...

Suggestion 🔍 Search Terms noImplicitAny @types Related: #33886 #27265 #26188 ✅ Viability Checklist My suggestion meets these guidelines: This wouldn't be a breaking change in existing TypeScrip...

TypeScript developers are a lucky bunch: for us, Christmas comes four times a year when the TypeScript team releases a new beta version. This is our opportunity to try out the latest features....

Software engineering is a battle against complexity. Without any planning or care, it's easy to build programs where everything interacts with everything else (the "big ball of yarn" model). With a...

I cut one item from Effective TypeScript during the final stages of editing. Four years later, it's time for it to see the light of day! It's a trick for specializing generic types for certain...

Every year I do the Advent of Code in a different programming language. If you aren't familiar, it's an online coding competition with a new two-part problem every day from December 1st to the 25th....

Rob Palmer shares some of the insights & lessons learned during Bloomberg Engineering's journey to adopt TypeScript as a first-class supported language.

This post looks at the Closure Compiler, Google's tool from the mid-2000s for adding types to JavaScript. It looks at how its focus on minification led to very different design choices than TypeScr...

Years ago when Brett Slatkin wrote Effective Python, he blogged about creating a tool call pyliterate to run all the code samples in his book and verify that their output matched what he'd written in...

I'd never describe problem sets as "fun," but there's a reason teachers assign them. It's one thing to nod your head as you read a book ("yes, that makes sense") but quite another to apply what...

TypeScript's infer keyword can infer quite a bit more than you might expect. It's extremely effective at extracting types from the sort of nested structures that you might get from codegen or an API...

Tuesday: A trip back in time, steamy weather, and your favorite almost-onion.

Conditional types are the most powerful weapon TypeScript gives us for mapping between types. They do their best work on union types, so sometimes it pays to apply slightly counterintuitive transfo...

TypeScript's type system has grown steadily more powerful over the past five years, allowing you to precisely type more and more patterns in JavaScript. The upcoming TypeScript 4.1 release includes a...

TypeScript tends to do a very good job of inferring types when you leave off explicit annotations. (Chapter 3 of Effective TypeScript is devoted to this topic.) But when you use tuples or string...

This series will present a few tips and tricks for working with TypeScript generics that I've picked up over the past year. This post looks at how you can use classes and currying to combine explicit...

The first thing that many new TypeScript developers do when they convert a codebase from JavaScript is fill it with type annotations. TypeScript is about types, after all! But in TypeScript many...

When you're working with generic types and run into a seemingly unavoidable error, there's a trick that can often make it go away: intersect what you have with whatever TypeScript wants it to be....

One of underscore.js’s most useful methods is _.pluck. It takes an array of objects and "plucks" one property out of each, returning an…

TypeScript's type system has grown steadily more powerful over the past five years, allowing you to precisely type more and more patterns in JavaScript. The upcoming TypeScript 4.1 release includes a...

Last year I wrote about Jsonify, a generic that models how a type changes as it goes through JSON serialization and deserialization. Since then I've learned two important things: 1) I didn't come up...

Chapter 4 of Effective TypeScript covers type design: the process of crafting your types to accurately model your domain. This item has always been a favorite of mine because of how immediately...

I'd like to improve editor interaction speed and the time that tsc takes to run over my TypeScript code. One of the standard ways to do this is with project references. There have been many reports...

The New TypeScript Handbook has some real gems in it. Here's what it has to say about generics: Writing generic functions is fun, and it can be easy to get carried away with type parameters. Having...