Setup TypeScript, Eslint, Jest and Prettier

Refer to here.


A variable can be declared to have multiple types. TypeScript will throw an error if the variable type might not fit an operation.

function repeat(n: string | number) {
  return "x".repeat(n);
  // [Error]
  // Argument of type 'string | number' is not assignable to parameter of type 'number'.

To eliminate the error, we can "narrow" down the type before the operation.

function repeat(x: string | number) {
  if (typeof n === "number") {
    // TypeScript will understand that `n` is a number.
    return "x".repeat(n);

  // TypeScript will understand that `n` is a string.
  return "x".repeat(Number(n));

Typescript supports the following narrowing guards:

  • Truthiness

  • Equality

  • typeof operator

  • in operator

  • instanceof operator

Please refer to the official documentation for the details.

Declaration Merging

Official Documentation

Declaration merging is an important technique to add custom fields to the existing interfaces.

Assume you are working with the HTTP server. If you want to add the logger to the req object as req.log, TypeScript will warn you that

Property 'log' does not exist on type 'IncomingMessage'.

To achieve that, you have to declare your own IncomingMessage interface to merge with the default http.IncomingMessage interface.

  1. Declare your custom interface - ./src/types/http/index.d.ts

    import { Logger } from "pino";
    declare module "http" {
      interface IncomingMessage {
        log: Logger;

    The folder path must be /path/to/<MODULE>/index.d.ts for ts-node to work properly.

  2. In tsconfig.json, update compilerOptions.typeRoots to include the custom "types" folder.

      // ...
      "compilerOptions": {
        // ...
        "typeRoots": ["./node_modules/@types", "./src/types"]
  3. Done. TypeScript will acknowledge that req.log is a Logger from now on.

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