Learn to test your packages NPM locally.

> src > index.ts

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

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

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

export function division(a: number, b: number): number {
  if (b < 0) throw new Error("Cannot divide by zero");
  return a / b;
}

> tsconfig.json

{
  "compilerOptions": {
    /* Visit https://aka.ms/tsconfig.json to read more about this file */

    /* Language and Environment */
    "target": "ES5",

    /* Modules */
    "module": "commonjs",                                /* Specify what module code is generated. */
    "rootDir": "./src",                                  /* Specify the root folder within your source files. */
    "baseUrl": "./src",

    /* Emit */
    "declaration": true,                              /* Generate .d.ts files from TypeScript and JavaScript files in your project. */
    "sourceMap": true,                                /* Create source map files for emitted JavaScript files. */
    "outDir": "./lib",                                   /* Specify an output folder for all emitted files. */

    /* Interop Constraints */
    "esModuleInterop": true,                         /* Disable resolving symlinks to their realpath. This correlates to the same flag in node. */
    "forceConsistentCasingInFileNames": true,            /* Ensure that casing is correct in imports. */

    /* Type Checking */
    "strict": true,                            /* Enable error reporting for expressions and declarations with an implied `any` type.. */
    "strictNullChecks": true,                         /* When type checking, take into account `null` and `undefined`. */
    "strictFunctionTypes": true,                            /* Ensure 'use strict' is always emitted. */
    "noUnusedLocals": true,                           /* Enable error reporting when a local variables aren't read. */
    "noUnusedParameters": true,                          /* Interpret optional property types as written, rather than adding 'undefined'. */
    "noImplicitReturns": true,                        /* Enable error reporting for codepaths that do not explicitly return in a function. */
    "skipLibCheck": true                                 /* Skip type checking all .d.ts files. */
  }
}

> package.json

{
  "name": "operations-basic",
  "version": "0.1.0",
  "description": "Little descriptions of what your package does",
  "main": "lib/index.js",
  "types": "lib",
  "scripts": {
    "build": "tsc -p ."
  },
  "keywords": ["Some keywords"],
  "author": {
    "name": "<Your name>",
    "email": "<Your email>",
    "url": "https://github.com/<Your username>"
  },
  "repository": {
    "url": "https://github.com/<Your username>/<Package name>"
  },
  "license": "MIT",
  "dependencies": {
  },
  "devDependencies": {
  }
}