Node.js Playground

This is my Node.js playground.

Run the following commands to run locally:

git clone https://github.com/pyk/node-playground.git
cd node-playground
npm install

There are a plugins for VS Code that useful:

ESLint. This plugin will helps you to catch a typo/error without running the code.

Const

In this section we will talk about const because it is commonly used in a Node.js codebase.

What is `const`?

const is a keyword to represent a constant value. A constant value is a value that never change, if the value is change then it raise an error.

Example

// const.js
const number = 10;

number = 2;

Run:

node const.js

It will throw an error:

TypeError: Assignment to constant variable.
    at Object.<anonymous> (/home/pyk/go/src/github.com/pyk/node-playground/const.js:3:8)
    at Module._compile (internal/modules/cjs/loader.js:654:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:665:10)
    at Module.load (internal/modules/cjs/loader.js:566:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:506:12)
    at Function.Module._load (internal/modules/cjs/loader.js:498:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:695:10)
    at startup (internal/bootstrap/node.js:201:19)

Modules

There are 3 different types of modules:

Core Modules. The core modules are defined within Node.js's source and are located in the lib/ folder. To load the core modules use the following syntax require($ID) where $ID is like 'http' or 'fs'.
File Modules. The file modules is a module that created by user. The module can be loaded as reuire('filename.js'). Only exported definition are available inside the caller.
Folders as Modules. The folder should contains file named index.js. The module can be loaded as require('foldername') where foldername/index.js is expected to exists.

Core Modules Example

const fs = require('fs');
const path = require('path');
const util = require('util');

// Promisify callback-based function
const readFile = util.promisify(fs.readFile);

const main = async () => {
  const filePath = path.join(__dirname, 'data.txt');
  // TODO: async/await node.js 9
  const data = await readFile(filePath, { encoding: 'utf8', flag: 'r' });
  console.log(data);
};

main();

File Modules Example

We create new file called math.js with the following content:

const add = (a, b) => a + b;
const sub = (a, b) => a - b;

// Exports the functions
exports.add = add;
exports.sub = sub;

Notice these two lines:

exports.add = add;
exports.sub = sub;

These lines are required in order to use add and sub functions outside the math.js.

Then we add new math.js consumer file named main.js with the following content:

const math = require('./math.js');

console.log(math.add(1, 2));
console.log(math.sub(4, 8));

Run the main.js, you will get the following output:

3
-4

Folder Modules Example

We create new file called some-lib/index.js with the same content as math.js.

Update the main.js to this:

// const math = require('./math.js');
const math = require('./some-lib');

console.log(math.add(1, 2));
console.log(math.sub(4, 8));

you should get the same output.

pyk/node-playground