环境变量

本文档适用于 9.4 和更高版本的 Next.js。如果你使用的是较旧版本的 Next.js,请参阅 next.config.js 中的环境变量

Examples

Next.js 内置了对环境变量的支持,让你可以执行以下操作:

Loading Environment Variables

Next.js has built-in support for loading environment variables from .env.local into process.env.

An example .env.local:

DB_HOST=localhost
DB_USER=myuser
DB_PASS=mypassword

This loads process.env.DB_HOST, process.env.DB_USER, and process.env.DB_PASS into the Node.js environment automatically allowing you to use them in Next.js data fetching methods and API routes.

For example, using getStaticProps:

// pages/index.js
export async function getStaticProps() {
  const db = await myDB.connect({
    host: process.env.DB_HOST,
    username: process.env.DB_USER,
    password: process.env.DB_PASS,
  })
  // ...
}

Note: Next.js will automatically expand variables ($VAR) inside of your .env* files. This allows you to reference other secrets, like so:

# .env
HOSTNAME=localhost
PORT=8080
HOST=http://$HOSTNAME:$PORT

If you are trying to use a variable with a $ in the actual value, it needs to be escaped like so: \$.

For example:

# .env
A=abc
WRONG=pre$A # becomes "preabc"
CORRECT=pre\$A # becomes "pre$A"

Exposing Environment Variables to the Browser

By default all environment variables loaded through .env.local are only available in the Node.js environment, meaning they won't be exposed to the browser.

In order to expose a variable to the browser you have to prefix the variable with NEXT_PUBLIC_. For example:

NEXT_PUBLIC_ANALYTICS_ID=abcdefghijk

This loads process.env.NEXT_PUBLIC_ANALYTICS_ID into the Node.js environment automatically. Allowing you to use it anywhere in your code. The value will be inlined into JavaScript sent to the browser because of the NEXT_PUBLIC_ prefix.

// pages/index.js
import setupAnalyticsService from '../lib/my-analytics-service'

// NEXT_PUBLIC_ANALYTICS_ID can be used here as it's prefixed by NEXT_PUBLIC_
setupAnalyticsService(process.env.NEXT_PUBLIC_ANALYTICS_ID)

function HomePage() {
  return <h1>Hello World</h1>
}

export default HomePage

Default Environment Variables

In general only one .env.local file is needed. However, sometimes you might want to add some defaults for the development (next dev) or production (next start) environment.

Next.js allows you to set defaults in .env (all environments), .env.development (development environment), and .env.production (production environment).

.env.local always overrides the defaults set.

Note: .env, .env.development, and .env.production files should be included in your repository as they define defaults. .env*.local should be added to .gitignore, as those files are intended to be ignored. .env.local is where secrets can be stored.

Environment Variables on Vercel

When deploying on Vercel you can configure secrets in the Environment Variables section of the project in the Vercel dashboard.

You can still use .env, .env.development and .env.production to add defaults.

If you've configured Development Environment Variables you can pull them into a .env.local for usage on your local machine using the following command:

vercel env pull .env.local

Test Environment Variables

Apart from development and production environments, there is a 3rd option available: test. In the same way you can set defaults for development or production environments, you can do the same with .env.test file for testing environment (though this one is not so common as the previous two).

This one is useful when running tests with tools like jest or cypress where you need to set specific environment vars only for testing purposes. Test default values will be loaded if NODE_ENV is set to test, though you usually don't need to do this manually as testing tools will address it for you.

There is a small difference between test environment, and both development and production that you need to bear in mind: .env.local won't be loaded, as you expect tests to produce the same results for everyone. This way every test execution will use same env defaults across different executions by ignoring your .env.local (which is intended to override the default set).

Note: similar to Default Environment Variables, .env.test file should be included in your repository, but .env.test.local shouldn't, as .env*.local are intended to be ignored through .gitignore.