How do I pass command line arguments to a Node.js program?

The Problem

How do we pass command-line arguments to a Node.js script or program, and how can we access them in Node.js?

The Solution

Command-line arguments are a great way to provide additional information and/or customization to a Node.js script.

Passing the Arguments

We can pass any number of command-line arguments while invoking a Node.js application by typing them in after the name of the file, like so:

node example.js a b c d

Arguments can be standalone or they can be a key-value pair:

node example.js letter=a

Retrieving the Arguments

Node.js has a built-in object called process. This object exposes an array argv, which contains all the arguments passed from the command-line.

But first, let’s take a look at what happens when we launch a Node.js process without any arguments.

Let’s create a file index.js and add the following code to it:

process.argv.forEach((value, index) => {
  console.log(index, value);
});

When we run the following command:

node index.js

We get the following output in our terminal:

0 \path\to\node.exe
1 \path\to\index.js

There are two things to note:

• The array argv exists even if we do not pass any additional arguments to our Node.js process.
• The first two elements of this array always contain the path to our node binary and the path to our script.

Let’s re-run the script, but this time with some arguments:

node index.js one two three env=dev

And this time we get:

0 \path\to\node.exe
1 \path\to\index.js
2 one
3 two
4 three
5 env=dev

In order to get only the additional arguments we can create a new array and exclude the first two elements like so:

const arguments = process.argv.slice(2);
arguments.forEach((value, index) => {
  console.log(index, value);
});

And we get the following:

0 one
1 two
2 three
3 env=dev