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

When running a Node.js program, you can supply additional command line arguments after the file name. For example:

node index.js arg1 arg2 arg3

Inside your Node.js script (e.g., index.js), you can read these arguments using process.argv, which is an array of strings:

  1. process.argv[0]: The path to the Node.js executable.
  2. process.argv[1]: The path to the script file (e.g., index.js).
  3. process.argv[2] onward: Actual command line arguments you passed.

1. Basic Usage

// index.js
const args = process.argv.slice(2); // start from index 2
console.log('Received arguments:', args);

If you run:

node index.js arg1 arg2 arg3

It will print:

Received arguments: [ 'arg1', 'arg2', 'arg3' ]

Why slice(2)?

  • Indices 0 and 1 represent paths to the Node binary and your script.
  • The real arguments start at index 2.

2. Handling Named Arguments

If you want to pass and parse named arguments (like --name=Alice), you can manually split strings or use a library (e.g., minimist, yargs).

Example with minimist:

  1. Install:

    npm install minimist

  2. Use:

    // index.js
const minimist = require('minimist');
const args = minimist(process.argv.slice(2));

console.log('Name:', args.name);
console.log('Age:', args.age);

  3. Run:

    node index.js --name=Alice --age=30

    Output:

    Name: Alice
Age: 30

3. Passing Arguments via npm Scripts

If you’re using npm run scripts in your package.json, you can pass additional arguments by placing -- after the script name:

{
  "scripts": {
    "start": "node index.js"
  }
}

Then in your terminal:

npm run start -- arg1 arg2

All arguments after -- will be forwarded to your Node.js script.

4. Best Practices and Tips

  1. Parse Arguments Carefully

    • If your application expects named options or flags (e.g., --debug, --port=3000), use a library like minimist or yargs to handle parsing consistently and reliably.

  2. Validate User Inputs

    • Don’t trust arguments blindly. Validate or sanitize arguments to avoid unexpected behavior.

  3. Check for Missing/Extra Arguments

    • If certain arguments are mandatory, display a help message or fallback defaults when they’re not provided.

  4. Documentation

    • Provide clear instructions or a --help option so users know how to supply and interpret command line arguments.

Recommended Resource

Summary

  • Use process.argv in Node.js to access command line arguments.
  • process.argv[2] onward contains the actual arguments you pass.
  • For more advanced parsing (named arguments, flags), a library like minimist or yargs can simplify the process.
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