JavaScript & TypeScript Tips

I already had a library that I wanted to convert to Typescript. I picked Rollup.js to do my build process. First I installed the following packages: > npm i -E -D rollup typescript @rollup/plugin-commonjs\ @rollup/plugin-node-resolve rollup-plugin-typescript2\ rollup-plugin-peer-deps-external I ended up with: * [email protected] * [email protected] * [email protected] * [email protected] * @rollup/[email protected] * @rollup/[email protected] Then I created a tsconfig.json: { "compilerOptions": { "outDir": "build", "module": "esnext", "target": "es5", "lib": ["es6", "dom", "es2016", "es2017"], "sourceMap": true, "allowJs": false, "declaration": true, "moduleResolution": "node", "forceConsistentCasingInFileNames": true, "noImplicitReturns": true, "noImplicitThis": true, "noImplicitAny": true, "strictNullChecks": true, "suppressImplicitAnyIndexErrors": true, "noUnusedLocals": true, "noUnusedParameters": true, "esModuleInterop": true, "allowSyntheticDefaultImports": true }, "include": ["src"], "exclude": ["node_modules", "build"] } Also a rollup.config.js: ...

I found different posts that tell you how to mock Axios using Jest & Typescript. The only difference in this post is that, when I use Axios, I like to use it as a function rather than calling axios.get or axios.post. Imagine you have this Axios request that you want to mock in your tests: //src/index.ts import axios from "axios"; export interface Post { userId: number; id: number; title: string; body: string; } const DummyRequest = (id: number): Promise<Post> => { return axios({ method: "GET", url: `https://jsonplaceholder.typicode.com/posts/${id}`, }).then((response) => { return { ...response.data }; }); }; export default DummyRequest; Install jest and jest-ts and initialize jest-ts ...

Last night I was reading this post: ES6 arrow functions, syntax and lexical scoping and going through the comments I saw this question: so arrow functions always inherit scope? The answer was by Barney: always. I went to the console and typed: nvm use 4 node var doSome = () => { console.log(this.x) } doSome.call({x: 'hello'}); global.x = 'hello'; doSome.call({x: 'good bye'}); Could you guess what is going to be printed? I could replicate this on ES5 without fat arrow function. ...