How can I use variables in package.json ?

Directly using variables in package.json is not supported because JSON is a static data format that does not support variables or execute any logical operations. However, we can indirectly achieve similar functionality through several methods. Here are a few approaches:

1. Using Environment Variables

Although you cannot directly define variables in the package.json file, you can use environment variables within npm scripts. For example, you can set environment variables when running npm scripts and reference them in the scripts section of package.json:

json
{
  "scripts": {
    "start": "PORT=3000 node app.js",
    "start:prod": "NODE_ENV=production node app.js"
  }
}

In this example, we set the PORT and NODE_ENV environment variables, which are used in different scripts.

2. Using npm Configuration Variables

npm allows you to set configuration variables via the command line, which can be used in npm scripts. Here's how to set it up:

bash
npm config set my-app:port 3000

Then reference this variable in package.json:

json
{
  "scripts": {
    "start": "node app.js --port=${npm_package_config_port}"
  }
}

3. Using External Configuration Files

You can create an external configuration file (e.g., config.json) and load it in your application. In the package.json scripts, you can write a small script to set or modify this configuration file before starting the application:

json
{
  "scripts": {
    "start": "node setupConfig.js && node app.js"
  }
}

Here, setupConfig.js is a simple Node.js script used to modify or generate the configuration file.

4. Using Custom Node Scripts for Processing

You can write a custom Node.js script to process specific fields in package.json, then run this script before executing the actual command:

json
{
  "scripts": {
    "prestart": "node preprocess.js",
    "start": "node app.js"
  }
}

In preprocess.js, you can read the package.json file, modify it as needed, save the changes, and proceed with other operations.

Summary

Although package.json itself does not support variables, we can achieve flexible variable usage in npm scripts by leveraging environment variables, npm configuration variables, external configuration files, or custom scripts. These methods bypass the static limitations of package.json by using external operations to implement dynamic configuration.