Environment Variables#

Rsbuild supports injecting environment variables or expressions into the code during compilation, which is helpful for distinguishing the running environment or replacing constants. This chapter introduces how to use environment variables.

Default Variables#

process.env.NODE_ENV#

By default, Rsbuild will automatically set the process.env.NODE_ENV environment variable to 'development' in development mode and 'production' in production mode.

You can use process.env.NODE_ENV directly in Node.js and in the client code.

if (process.env.NODE_ENV === 'development') {
  console.log('this is a development log');
}

In the development build, the above code will be compiled as:

if (true) {
  console.log('this is a development log');
}

In the production build, the above code will be compiled as:

if (false) {
  console.log('this is a development log');
}

After code minification, if (false) { ... } will be recognized as invalid code and removed automatically.

process.env.ASSET_PREFIX#

You can use process.env.ASSET_PREFIX in the client code to access the URL prefix of static assets.

• In development, it is equivalent to the value set by dev.assetPrefix.
• In production, it is equivalent to the value set by output.assetPrefix.
• Rsbuild will automatically remove the trailing slash from assetPrefix to make string concatenation easier.

For example, we copy the static/icon.png image to the dist directory through output.copy configuration:

export default {
  dev: {
    assetPrefix: '/',
  },
  output: {
    copy: [{ from: './static', to: 'static' }],
    assetPrefix: 'https://example.com',
  },
};

Then we can access the image URL in the client code:

const Image = <img src={`${process.env.ASSET_PREFIX}/static/icon.png`} />;

In the development environment, the above code will be compiled as:

const Image = <img src={`/static/icon.png`} />;

In the production environment, the above code will be compiled as:

const Image = <img src={`https://example.com/static/icon.png`} />;

.env File#

When a .env file exists in the project root directory, Rsbuild will automatically use dotenv to load these environment variables and add them to the current Node.js process.

File Types#

Rsbuild supports reading the following types of env files:

If several of the above files exist at the same time, they will all be loaded, with the files listed at the bottom of the table having higher priority.

Env Mode#

Rsbuild also supports reading .env.[mode] and .env.[mode].local files. You can specify the env mode using the --env-mode <mode> CLI option.

For example, set the env mode as test:

npx rsbuild dev --env-mode test

Rsbuild will then read the following files in sequence:

• .env
• .env.local
• .env.test
• .env.test.local

Example#

For example, create a .env file and add the following contents:

FOO=hello
BAR=1

Then in the rsbuild.config.ts file, you can directly access the above environment variables:

console.log(process.env.FOO); // 'hello'
console.log(process.env.BAR); // '1'

Now, create a .env.local file and add the following contents:

BAR=2

The value of process.env.BAR is overwritten to '2':

console.log(process.env.FOO); // 'hello'
console.log(process.env.BAR); // '2'

Public Variables#

All environment variables starting with PUBLIC_ can be accessed in client code. For example, if the following variables are defined:

PUBLIC_NAME=jack
PASSWORD=123

In the source file of the client code, you can access public variables in the following way. Rsbuild will replace identifiers starting with process.env.PUBLIC_ with the corresponding expression.

console.log(process.env.PUBLIC_NAME); // -> 'jack'
console.log(process.env.PASSWORD); // -> undefined

Replacement Scope#

Public variables will replace identifiers in the client code, with the replacement scope including:

• JavaScript files, and files that can be converted into JavaScript code, such as .js, .ts, .tsx, etc.
• HTML template files, for example:

<div><%= process.env.PUBLIC_NAME %></div>

Note that public variables will not replace identifiers in the following files:

• CSS files, such as .css, .scss, .less, etc.

Custom Prefix#

Rsbuild provides the loadEnv method, which can inject environment variables with any prefix into client code.

For example, when migrating a Create React App project to Rsbuild, you can read environment variables starting with REACT_APP_ and inject them through the source.define config as follows:

import { defineConfig, loadEnv } from '@rsbuild/core';

const { publicVars } = loadEnv({ prefixes: ['REACT_APP_'] });

export default defineConfig({
  source: {
    define: publicVars,
  },
});

Using define config#

By configuring the source.define, you can replace expressions with other expressions or values in compile time.

Define looks like macro definitions in other programming languages. But JavaScript has powerful runtime capabilities, so you don't need to use it as a complicated code generator. You can use it to pass simple data, such as environment variables, from compile time to client code. Almost there, it can be used to work with Rsbuild to shake trees.

Replace Expressions#

The most basic use case for Define is to replace expressions in compile time.

The value of the environment variable NODE_ENV will change the behavior of many vendor packages. Usually, we need to set it to production.

export default {
  source: {
    define: {
      'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV),
    },
  },
};

Note that the value provided here must be a JSON string, e.g. process.env.NODE_ENV with a value of "production" should be passed in as "\"production\"" to be processed correctly.

Similarly { foo: "bar" } should be converted to "{\"foo\":\"bar\"}", which if passed directly into the original object would mean replacing the expression process.env.NODE_ENV.foo with the identifier bar.

For more about source.define, just refer to API References.

process.env Replacement#

When using source.define, please avoid replacing the entire process.env object, e.g. the following usage is not recommended:

export default {
  source: {
    define: {
      'process.env': JSON.stringify(process.env),
    },
  },
};

If the above usage is adopted, the following problems will be caused:

1. Some unused environment variables are additionally injected, causing the environment variables of the development environment to be leaked into the front-end code.
2. As each process.env code will be replaced by a complete environment variable object, the bundle size of the front-end code will increase and the performance will decrease.

Therefore, please inject the environment variables on process.env according to actual needs and avoid replacing them in its entirety.

Notes#

Note that source.define will only match the full expression; destructing the expression will prevent the Rsbuild from correctly recognizing it.

console.log(process.env.NODE_ENV);
// => production

const { NODE_ENV } = process.env;
console.log(NODE_ENV);
// => undefined

const vars = process.env;
console.log(vars.NODE_ENV);
// => undefined

Declare type of environment variable#

When you read an environment variable in a TypeScript file, TypeScript may prompt that the variable lacks a type definition, and you need to add the corresponding type declaration.

For example, if you reference a CUSTOM_VAR variable, the following prompt will appear in the TypeScript file:

To fix this, you can create a src/env.d.ts file in your project and add the following content:

declare const CUSTOM_VAR: string;

Tree Shaking#

Define can also be used to mark dead code to assist the Rsbuild with Tree Shaking optimization.

Build artifacts for different regions is achieved by replacing process.env.REGION with a specific value, for example.

export default {
  source: {
    define: {
      'process.env.REGION': JSON.stringify(process.env.REGION),
    },
  },
};

For an internationalized app:

const App = () => {
  if (process.env.REGION === 'cn') {
    return <EntryFoo />;
  } else if (process.env.REGION === 'sg') {
    return <EntryBar />;
  } else {
    return <EntryBaz />;
  }
};

Specifying the environment variable REGION=sg and then running build will eliminate any dead code.

const App = () => {
  if (false) {
  } else if (true) {
    return <EntryBar />;
  } else {
  }
};

Unused components are not bundled into the artifacts, and their external dependencies can be optimized accordingly, resulting in a destination with better size and performance.