`satisfies` is my favorite TypeScript keyword (2024)

Post by mckirk » Sun, Nov 23, 2025, 2:25 AM UTC

amake wrote: post #48

99% of my use of `satisfies` is to type-check exhaustivity in `switch` statements: type Foo = 'foo' | 'bar'; const myFoo: Foo = 'foo'; switch (myFoo) { case 'foo': // do stuff break; default: myFoo satisfies never; // Error here because 'bar' not handled }

Post by RHSeeger » Sun, Nov 23, 2025, 2:34 AM UTC

reissbaker wrote: post #17

ptrl600 wrote: post #6

This is wordier than just "as const", what advantage does it give? (I am a newbie and genuinely don't know) edit: perhaps the advantage only comes into play for mutable values, where you want a narrower type than default, but not that narrow. Indeed, this is covered in the article, but CTRL+F "as const" doesn't work on the page for whatever reason, so I missed it.

The satisfies keyword is quite different than "as const." What it does is: 1. Enforce that a value adheres to a specific type 2. But, doesn't cause the value to be cast to that type. For example, if you have a Rect type like: type Rect = { w: number, h: number } You might want to enforce that some value satisfies Rect properties... But also allow it to have others. For example: const a = { x: 0, y: 0, w: 5, h: 5 }; I…

    function coolPeopleOnly(person: Person & { isCool: true }) {
        // only cool people can enter here
    }

    const person = {
        name: "Jerred",
       isCool: true,
    } satisfies Person;

    coolPeopleOnly(person);

Post by geoffmanning » Sun, Nov 23, 2025, 2:37 AM UTC

wk_end wrote: post #68

spankalee wrote: post #61

Earlier quoted context omitted.

We don't have to deal in hypotheticals - we have a concrete example here. There's a method, array.flat() that does a thing that we can correctly describe in TypeScript's type system. You say you would reject those correct types, but for what alternative? It's hugely beneficial to library users to automatically get correctly type return values from functions without having to do error-prone casts. I would always take…

There's nothing I can do about the standard JavaScript library, but in terms of code I have influence over, I very simply would not write a difficult-to-type method like Array.prototype.flat(), if I could help it. That's what I mean by an XY Problem - why are we writing this difficult-to-type method in the first place and what can we do instead? Let's suppose Array.prototype.flat() wasn't in the standard library, whi…

Post by coolThingsFirst » Sun, Nov 23, 2025, 3:06 AM UTC

Post by fenomas » Sun, Nov 23, 2025, 3:17 AM UTC

dzonga wrote: post #49

epolanski wrote: post #3

> TypeScript is a wonderfully advanced language though it has an unfortunately steep learning curve An extremely steep one. The average multi-year TypeScript developer I meet can barely write a basic utility type, let alone has any general (non TypeScript related) notion of cardinality or sub typing. Hell, ask someone to write a signature for array flat, you'd be surprised how many would fail. Too many really stop at…

typescript is largely a result of solving a non-existent problem. Yeah JS is finicky & has foot-guns, however they're ways around those foot guns that don't involve typescript. Rich Hickey in 10 Years of Clojure & Maybe Not then the Value of Values - lays this out - though not meant at typescript but static types in general. the thing most people don't have proper Javascript fundamentals. Function signatures: JSDoc w…