NodeJS

Install argparse using

npm i --save argparse

Optional argument:

const ArgumentParser = require('argparse').ArgumentParser;
const parser = new ArgumentParser({});
parser.addArgument(['-u', '--url'], {help: "The URL to use"});
const args = parser.parseArgs();

// Example usage
console.log(args.url) // null if no such argument

If you want to make an argument mandatory, use required: true:

const ArgumentParser = require('argparse').ArgumentParser;
const parser = new ArgumentParser({});
parser.addArgument(['-u', '--url'], {help: "The URL to use", required: true});
const args = parser.parseArgs();

// Example usage
console.log(args.url)

In case the user does not provide the argument, it will print

usage: CLI.js [-h] -u URL
CLI.js: error: Argument "-u/--url" is required

Positional arguments

This positional argument will always be required:

const ArgumentParser = require('argparse').ArgumentParser;
const parser = new ArgumentParser({});
parser.addArgument(['url'], {help: "The URL to use"});
const args = parser.parseArgs();

// Example usage
console.log(args.url)

Use this function to determine if the current NodeJS process is being run by the root user:

function isCurrentUserRoot() {
    return process.getuid() == 0; // UID 0 is always root
}

Usage example:

if(isCurrentUserRoot()) {
    // TODO Your code for root user goes here
} else {
    // TODO Your code for NON-root user goes here!
}

This works since the UID of the root user is always 0 on Linux/Unix systems.

Note: You don’t need to

const process = require("process");

since the process object is automatically imported in any NodeJS environment

You can use process.env.NODE_ENV to check the value of the NODE_ENV environment variable in Node.JS:

if(process.env.NODE_ENV == "development") {
    // TODO your code goes here
}

or

if(process.env.NODE_ENV == "production") {
    // TODO your code goes here
}

Note that by default NODE_ENV is undefined so remember to handle that case appropriately.

This is the minimal Koa.JS application which I use as a template for new NodeJS webserver applications.

#!/usr/bin/env node
const router = require('koa-router')();
const koaBody = require('koa-body');
const Koa = require('koa');
const app = new Koa();

app.use(koaBody());

router.get('/', async ctx => {
    ctx.body = "Hello world";
});

app.use(router.routes());

if (!module.parent) app.listen(3000);

Install the requirements using

npm i --save koa koa-router koa-body

and run using

node index.js

assuming you have saved our code from above in index.js.

Now (with the node index.js command still running) go to http://localhost:3000 . You should see Hello world there. Now it’s your turn to continue on your endeavour to develop the world’s greatest webservers 🙂

In NodeJS applications you often see code like

if (!module.parent) {
    app.listen(3000);
}

This means: Run app.listen(3000) only if you are running the file

Suppose this code is in index.js. In this case, the code will only be executed if you run index.js directly (i.e. using node index.js) and not if index.js is required from another file (by require('./index.js');).

If index.js is required from another Javascript module (i.e. file), module.parent will be set to that module.

The easiest way to sleep using async/await in NodeJS is the sleep-promise package:

npm install --save sleep-promise

Using that package you can simply use await sleep(milliseconds) syntax like this:

const sleep = require('sleep-promise');

// In any async function:
await sleep(2000); // Wait 2000 ms

Note that this sleep() variant is fully asynchronous, IO and other asynchronous operations will still be able to continue in the background – sleep() will not block the NodeJS process.

Full example:

const sleep = require('sleep-promise');

(async () => {
    console.log("This prints immediately");
    await sleep(2000);
    console.log("This prints 2 seconds later");
})();

Install the sleep-promise package:

npm install --save sleep-promise

then you can use

const sleep = require('sleep-promise');

// In any async function:
await sleep(2000); // Wait 2000 ms

Full example:

const sleep = require('sleep-promise');

(async () => {
    console.log("This prints immediately");
    await sleep(2000);
    console.log("This prints 2 seconds later");
})();

Problem:

You want to run a NodeJS app using

npm start

but you only see this error message:

npm ERR! missing script: start

Solution:

You need to tell npm what to do when you run npm start explicitly by editing package.json.

First, identify the main file of your application. Most often it is called index.js, server.js or app.js. When you open package.json in an editor, you can also often find a line like

"main": "index.js",

In this example, index.js is the main file of the application.

Now we can edit package.json to add a start script.

In package.json, find the "scripts" section. If you have a default package.json, it will look like this:

"scripts": {
  "test": "echo \"Error: no test specified\" && exit 1"
},

Now add a comma to the end of the "test" line and add this line after it:

"start": "node index.js"

and replace index.js by the main file of your application (e.g. app.js, server.js, index.js etc).

The "scripts" section should now look like this:

"scripts": {
  "test": "echo \"Error: no test specified\" && exit 1",
  "start": "node index.js"
},

Now save and close package.json and run

npm start

to start your application.

You want to run a command like file my.pdf using NodeJS child-process.exec and get its stdout after it’s finished.

Solution:

TL;DR: (await exec('file my.pdf')).stdout

We’re using child-process-promise here in order to simplify our implementation. Install it using npm i --save child-process-promise !

const { exec } = require('child-process-promise');

async function run () {
    const ret = await exec(`file my.pdf`);
    return ret.stdout;
}

run().then(console.log).catch(console.error);

You can also use .stderr instead of .stdout to get the stderr output as a string

Problem:

You are trying to build your webpack project, but you see an error message like this:

/home/uli/project/node_modules/webpack-cli/bin/config-yargs.js:89
                                describe: optionsSchema.definitions.output.properties.path.description,
                                                                           ^

TypeError: Cannot read property 'properties' of undefined
    at module.exports (/home/uli/project/node_modules/webpack-cli/bin/config-yargs.js:89:48)
    at /home/uli/project/node_modules/webpack-cli/bin/webpack.js:60:27
    at Object.<anonymous> (/home/uli/project/node_modules/webpack-cli/bin/webpack.js:515:3)
    at Module._compile (internal/modules/cjs/loader.js:723:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:734:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)
    at Module.require (internal/modules/cjs/loader.js:659:17)
    at require (internal/modules/cjs/helpers.js:22:18)

Solution:

This is a known bug in webpack 4.20.0 – you can circumvent this issue by using webpack 4.19.0.

Look for a line like

"webpack": "^4.7.0",

in your package.json. The caret (^) allows npm to use any 4.x.x version – including the broken 4.20.0.

Replace the aforementioned line by

"webpack": "4.19.0",

to use only webpack 4.19.0.

After that, run npm install and retry building your application.