DefinePlugin

The DefinePlugin allows you to create global constants which can be configured at compile time. This can be useful for allowing different behavior between development builds and production builds. If you perform logging in your development build but not in the production build you might use a global constant to determine whether logging takes place. That's where DefinePlugin shines, set it and forget it rules for development and production builds.

new webpack.DefinePlugin({
  // Definitions...
});

Usage

Each key passed into DefinePlugin is an identifier or multiple identifiers joined with ..

• If the value is a string it will be used as a code fragment.
• If the value isn't a string, it will be stringified (including functions).
• If the value is an object all keys are defined the same way.
• If you prefix typeof to the key, it's only defined for typeof calls.

The values will be inlined into the code allowing a minification pass to remove the redundant conditional.

new webpack.DefinePlugin({
  PRODUCTION: JSON.stringify(true),
  VERSION: JSON.stringify('5fa3b9'),
  BROWSER_SUPPORTS_HTML5: true,
  TWO: '1+1',
  'typeof window': JSON.stringify('object'),
  'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV)
});

console.log('Running App version ' + VERSION);
if(!BROWSER_SUPPORTS_HTML5) require('html5shiv');

if (!PRODUCTION) {
  console.log('Debug info');
}

if (PRODUCTION) {
  console.log('Production log');
}

After passing through webpack with no minification results in:

if (!true) {
  console.log('Debug info');
}
if (true) {
  console.log('Production log');
}

and then after a minification pass results in:

console.log('Production log');

Feature Flags

Enable/disable features in production/development build using feature flags.

new webpack.DefinePlugin({
  'NICE_FEATURE': JSON.stringify(true),
  'EXPERIMENTAL_FEATURE': JSON.stringify(false)
});

Service URLs

Use a different service URL in production/development builds:

new webpack.DefinePlugin({
  'SERVICE_URL': JSON.stringify('https://dev.example.com')
});

Runtime values via runtimeValue

function (getterFunction, [string]) => getterFunction()

It is possible to define variables with values that rely on files and will be re-evaluated when such files change in the file system. This means webpack will rebuild when such watched files change.

Arguments:

• The first argument of the webpack.DefinePlugin.runtimeValue is a function that should return the value to be assigned to the definition.
• The second argument is an array of file paths to watch for. Pass true instead of [string] here to flag the module as uncacheable.

const fileDep = path.resolve(__dirname, 'sample.txt');

new webpack.DefinePlugin({
  BUILT_AT: webpack.DefinePlugin.runtimeValue(Date.now, [fileDep])
});

The value of BUILT_AT would be the time at which the 'sample.txt' was last updated in the file system, e.g. 1597953013291.