Jest is one of many famous testing frameworks in Typescript

Jest by default runs in CommonJS mode and doesn’t understand TypeScript imports unless we configure a transformer.

• Initialize a simple TS project

npm init -y
npx tsc --init

• Change rootDir and srcDir

"rootDir": "./src",
"outDir": "./dist",

• Create src/index.ts

export function sum(a: number, b: number) {
    return a + b
}

export function multiply(a: number, b: number) {
    return a * b
}

export function substract(a: number, b: number) {
    return a - b
}

export function devide(a: number, b: number) {
    if (b === 0) throw Error("cant devide by zero")
    return a / b
}

• Add Ts-jest as a dependency

Jest was originally made for JavaScript app testing not so compatible with Ts but still we can use it with the help of Ts-jest

npm install --save-dev ts-jest  @jest/globals  @types/jest

• Initialize jest.config.ts

npx ts-jest config:init

update jest.config.js

import {createDefaultPreset} from "ts-jest"

const tsJestTransformCfg = createDefaultPreset().transform;

/** @type {import("jest").Config} **/
export default {
  testEnvironment: "node",
  preset: 'ts-jest',
  transform: {
    ...tsJestTransformCfg,
  },
};

• Update package.json