Explained: 'React' refers to a UMD global
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As an industry, we've started to agree that TypeScript is a good idea. It's a great way to catch bugs in your application code before they get to production.
But in many codebases, a huge part of your code isn't your application code. It's your tests. And in those tests, using TypeScript can be pretty painful.
Tests don't need to live up to the same level of strictness as the rest of your application code.
If you've got a broken type in your app, that'll likely end up in your users finding a bug. But what happens if your tests are passing, but you've got a broken type?
That broken type might be an indication that your tests are wrong - so you should probably check it. But once you've verified that it's all OK, it can feel like meaningless busywork.
Sometimes, your tests will need to test that when a function receives the wrong thing, it behaves in the right way.
Often the only way to achieve this is with an as any
:
typescript
const func = (a: string): void => {if (typeof a !== "string") {throw new Error("Oh dear!");}};it("Throws an error when passed a number", () => {expect(() => func(123 as any)).toThrow();});
You might have an ESLint rule enabled to prevent 'any' from your tests. If you do, you might look for the as unknown as string
pattern:
typescript
it("Throws an error when passed a number", () => {expect(() =>func(123 as unknown as string)).toThrow();});
This is extremely un-ergonomic - a DOUBLE assertion, just to be able to do something extremely common. If you've got experience with heavily tested codebases, you'll know how common these assertions are.
Another extremely common pattern is testing a function which receives a lot of properties, but you only care about a few of them. This can happen often with external libraries, such as express
endpoints:
typescript
import { Request, Response } from "express";const handler = (req: Request,res: Response): void => {};
In this case, you might only care about the req.body
property. But you still need to pass the entire Request
type to the function. So, you'll end up doing as unknown as Request
a lot.
Not only is this annoying, but you also need to import the type you want to use everywhere you perform the test. A thousand tiny cuts.
shoehorn
I've been through dozens of open-source application codebases, and all of them who used a large test suite had this problem. So, I've created a solution.
I've released a library, shoehorn
, for easing the pain of working with tests in TypeScript.
It gives you a first-class way to pass the wrong type to a function.
typescript
import { fromPartial } from "@total-typescript/shoehorn";it("Should get the user", () => {handler(// Mock the request!fromPartial({body: {id: "123",},}),// Mock the response!fromPartial({}));});
You can install shoehorn
from npm, and check out the GitHub repo.
I can't wait to see how you use it!
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Learn the differences between React.ReactNode
and JSX.Element
in TypeScript when working with React.
Since I first got into advanced TypeScript, I've been in love with a particular pattern. It formed the basis for one of my first-ever TypeScript tips, and it's been extraordinarily useful to me ever since. I call it the IIMT (rhymes with 'limped'): the Immediately Indexed Mapped Type.
There are three rules to keep in mind when deciding where to put types in your application code.
Discover the power of ComponentProps in React and TypeScript. Learn how to extract and utilize props from native HTML elements, existing components, and elements with associated refs. Enhance your development workflow with this essential type helper.
Testing types is a crucial aspect of developing libraries in TypeScript. In this article, we explore three different ways to test your types: using the vitest test runner, rolling your own solution with type helpers, and leveraging the tsd library.
The TypeScript 5.1 beta is out - here's everything you need to know.
There’s a difference between using TypeScript and knowing TypeScript.
The docs give you a good grasp of the pieces like generic functions, conditional types, and type helpers.
But out in the wild, developers are combining these pieces together into patterns.
Four of the most important patterns to know and use are:
The article discusses why TypeScript does not throw an error when a function that is assigned to a variable doesn't match its type. It explains that a function with fewer parameters than its type can still be passed, and this behavior is not restricted to TypeScript but exists in JavaScript as well.
TypeScript 5.0 introduces const type parameters which are useful in preserving the literal types of objects passed to functions.
Updates to TypeScript 5.0 have made their way into Total TypeScript!
Exclude
is a very powerful utility type that can be used in a variety of ways. In this article, I'll show you 9 ways to use it along with code examples.
As a frontend developer, your job isn't just pixel-pushing. Most of the complexity in frontend comes from handling all the various states your app can be in.
It might be loading data, waiting for a form to be filled in, or sending a telemetry event - or all three at the same time.
If you aren't handling your states properly, you're likely to come unstuck. And handling states starts with how th
Using the satisfies keyword is one of four ways to make type assignments in TypeScript. In this article we'll look at examples of when each method should be used.
Understand why TypeScript throws complicated errors by learning how to read them. Errors mirror the structure of the code being compared and can be simplified by changing the way types are assigned.
Learn how to use TypeScript generics on the type level and with functions to improve code readability, type safety, and reduce repetitive code. Use "type helpers" to create new types and generic functions to pass in and return specific types.
Use Zod to validate unknown inputs in your app, whether it's a CLI or a public API, to ensure that data entering your app is safe. Zod can also be used for 'sort of' trusted inputs, like third-party services, to ensure that the data returned is what you expect.
TypeScript's template literal syntax allows developers to manipulate and transform strings in a powerful way. This can be extended using unions, infer, and recursion to handle more complex tasks.
Donny (kdy1 on GitHub) is rewriting TypeScript in Rust hoping to speed up tsc which is slow due to its TypeScript base. In this interview, he explains some of the financial and technical challenges they face with this open-source project.
Let's imagine you're creating a function which sums up an array of objects. Here's one, taken from the Excalidraw codebase:
const sum = <T>(array: readonly T[], mapper: (item: T) => number): number => array.reduce((acc, item) => acc + mapper(item), 0)
Let's look at the type definition. This function takes in:
readonly T[]