‘Feature flags’ are variables that allow you to prevent specific code in the Gutenberg project from being shipped to WordPress core, and to run certain experimental features only in the plugin.

Introducing globalThis.IS_GUTENBERG_PLUGIN

The globalThis.IS_GUTENBERG_PLUGIN is an environment variable whose value ‘flags’ whether code is running within the Gutenberg plugin.

When the codebase is built for the plugin, this variable will be set to true. When building for WordPress core, it will be set to false or undefined.

Basic usage

Exporting features

A plugin-only function or constant should be exported using the following ternary syntax:

function myPluginOnlyFeature() {
    // implementation
}

export const pluginOnlyFeature = globalThis.IS_GUTENBERG_PLUGIN
    ? myPluginOnlyFeature
    : undefined;

In the above example, the pluginOnlyFeature export will be undefined in non-plugin environments such as WordPress core.

Importing features

If you’re attempting to import and call a plugin-only feature, be sure to wrap the function call in an if statement to avoid an error:

import { pluginOnlyFeature } from '@wordpress/foo';

if ( globalThis.IS_GUTENBERG_PLUGIN ) {
    pluginOnlyFeature();
}

How it works

During the webpack build, instances of globalThis.IS_GUTENBERG_PLUGIN will be replaced using webpack’s define plugin.

For example, in the following code –

if ( globalThis.IS_GUTENBERG_PLUGIN ) {
    pluginOnlyFeature();
}

– the variable globalThis.IS_GUTENBERG_PLUGIN will be replaced with the boolean true during the plugin-only build:

if ( true ) {
    // Webpack has replaced `globalThis.IS_GUTENBERG_PLUGIN` with `true`
    pluginOnlyFeature();
}

This ensures that code within the body of the if statement will always be executed.

In WordPress core, the globalThis.IS_GUTENBERG_PLUGIN variable is replaced with undefined. The built code looks like this:

if ( undefined ) {
    // Webpack has replaced `globalThis.IS_GUTENBERG_PLUGIN` with `undefined`
    pluginOnlyFeature();
}

undefined evaluates to false so the plugin-only feature will not be executed.

Dead code elimination

For production builds, webpack ‘minifies’ the code, removing as much unnecessary JavaScript as it can.

One of the steps involves something known as ‘dead code elimination’. For example, when the following code is encountered, webpack determines that the surrounding if statement is unnecessary:

if ( true ) {
    pluginOnlyFeature();
}

The condition will always evaluate to true, so webpack removes it, leaving behind the code that was in the body:

pluginOnlyFeature(); // The `if` condition block has been removed. Only the body remains.

Similarly, when building for WordPress core, the condition in the following if statement always resolves to false:

if ( undefined ) {
    pluginOnlyFeature();
}

In this case, the minification process will remove the entire if statement including the body, ensuring plugin-only code is not included in WordPress core build.

Frequently asked questions

Why shouldn’t I assign the result of an expression involving IS_GUTENBERG_PLUGIN to a variable, e.g. const isMyFeatureActive = ! Object.is( undefined, globalThis.IS_GUTENBERG_PLUGIN )?

Introducing complexity may prevent webpack’s minifier from identifying and therefore eliminating dead code. Therefore it is recommended to use the examples in this document to ensure your feature flag functions as intended. For further details, see the Dead Code Elimination section.