First Steps

Learn by example how to use TSTyche.

Prerequisites

The minimum supported Node.js version is 20.9. It is recommended to use the latest LTS release .

Install TSTyche

Use your favorite package manager to add tstyche (and optionally typescript) to a project:

npm add -D tstyche typescript

yarn add -D tstyche typescript

pnpm add -D tstyche typescript

It is recommended to create a tsconfig.json file as well. Run tsc --init and TypeScript will generate it.

Define a Type

Create a MethodLikeKeys.ts file with the following content:

type MethodLike = (...args: any) => any;
 
export type MethodLikeKeys<T> = keyof {
  [K in keyof T as T[K] extends MethodLike ? K : never]: T[K];
};

The MethodLikeKeys utility should construct a union type by picking up names of method like keys of the T type.

Add a Type Test

Create a MethodLikeKeys.tst.ts file in the same directory:

import { expect } from "tstyche";
import type { MethodLikeKeys } from "./MethodLikeKeys.js";
 
interface Sample {
  description: string;
  getLength: () => number;
  getWidth?: () => number;
}
 
expect<MethodLikeKeys<Sample>>().type.toBe<"getLength" | "getWidth">();

Here the expect() function defines an easy to read assertion. There are two method like keys in the Sample interface, hence the resulting type must be "getLength" | "getWidth".

Run the Type Test

All is set. Tell TSTyche to run the type test:

npx tstyche

yarn tstyche

pnpm tstyche

You should see similar report:

uses TypeScript 5.8.3 with ./tsconfig.json
 
fail ./MethodLikeKeys.tst.ts
 
Error: Type '"getLength"' is not the same as type '"getLength" | "getWidth"'.
 
   8 | }
   9 |
  10 | expect<MethodLikeKeys<Sample>>().type.toBe<"getLength" | "getWidth">();
     |                                            ~~~~~~~~~~~~~~~~~~~~~~~~
  11 |
 
       at ./MethodLikeKeys.tst.ts:10:44

On the first line you see the TypeScript version used for this test run. It is followed by a path to the TSConfig file from which the compiler configuration was loaded.

The rest reports that the test has failed and explains the problem.

Iterate

The getWidth key is marked as optional and was not picked up by the MethodLikeKeys type. Here is the fixed type:

export type MethodLikeKeys<T> = keyof {
  [K in keyof T as Required<T>[K] extends MethodLike ? K : never]: T[K];
};

Run the test again and it will pass:

uses TypeScript 5.8.3 with ./tsconfig.json
 
pass ./MethodLikeKeys.tst.ts