Environment Variables#

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

Default Variables#

process.env.NODE_ENV#

By default, Builder 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 runtime code.

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

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

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

In the production environment, 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 runtime 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.
• Builder 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 runtime 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`} />;

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 runtime. Almost there, it can be used to work with Builder 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 Injection#

When using source.define or source.globalVars, please avoid injecting 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.

So please avoid full injection, just inject the used variables from process.env.

Setup Environment Variables#

You may often need to set environment variables, in which case you can instead use the source.globalVars configuration to simplify configuration. It is a syntax sugar of source.define, the only difference is that source.globalVars will automatically stringify the value, which makes it easier to set the value of global variables and avoid writing a lot of JSON.stringify(...) stuffs.

export default {
  source: {
    globalVars: {
      'process.env.NODE_ENV': 'production',
      'import.meta.foo': { bar: 42 },
      'import.meta.baz': false,
    },
  },
};

Note that either of these methods will only match the full expression; destructing the expression will prevent the Builder 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 Builder 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),
    },
    // or...
    globalVars: {
      'process.env.REGION': 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 executing 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.

In-source testing#

Vitest supports writing tests inside source files to test the behavior of private features without exporting them. Set up Define to remove the test code from the production build. Please refer to the Vitest's documentation for detailed guidelines

// the implementation
function add(...args) {
  return args.reduce((a, b) => a + b, 0);
}

// in-source test suites
if (import.meta.vitest) {
  const { it, expect } = import.meta.vitest;
  it('add', () => {
    expect(add()).toBe(0);
    expect(add(1)).toBe(1);
    expect(add(1, 2, 3)).toBe(6);
  });
}