The enhance API provided by fusion-core allows for composing the value of Fusion.js tokens. With it, you can extend the behavior of any registered token to add or modify the default behavior of a token. Let's take a look at a simple example to understand how this works.

Let's create a simple plugin that just exports a configuration object:

import {createPlugin, createToken} from 'fusion-core';

export const ConfigurationToken = createToken('ConfigurationToken');

export default createPlugin({
  provides: () => ({
    databaseUserName: 'root',

We'll register that into our main app:

import ConfigurationPlugin, {ConfigurationToken} from './plugins/configuration';

const app = new App();
app.register(ConfigurationToken, ConfigurationPlugin);

Now we have a plugin registered to the ConfigurationToken token. If we were to depend on that token anywhere in the dependency graph, the value of that token would be an object with databaseUserName as the only key.

Using enhance however, we can extend the exported provides value and modify it. For example, we could add inject a dynamic configuration key so that all dependencies could access it:

import ConfigurationPlugin, {ConfigurationToken} from './plugins/configuration';

const app = new App();
app.register(ConfigurationToken, ConfigurationPlugin);
app.enhance(ConfigurationToken, (providedValue) => {
  return {
    databasePassword: '123456!',

Now, any other plugin that uses this token will receive the new provided value:

import {createPlugin} from 'fusion-core';
import {ConfigurationToken} from './configuration';

export default createPlugin({
  deps: {
    // This is the enhanced value e.g.
    // {
    //   databaseUserName: 'root',
    //   databasePassword: '123456!',
    // }
    configuration: ConfigurationToken,

More importantly, because tokens can be registered to plugins, we can wrap the original plugin with another plugin, thus extending functionality!

For example, if we had ExamplePlugin registered to ExampleToken, we could wrap the original plugin with a brand new created plugin:

app.enhance(ExampleToken, (examplePlugin) => {
  // Replace the original plugin bound to ExampleToken with a new plugin
  return createPlugin({
    // The new plugin provides the old plugin so that it is usable within this new plugin
    provides: examplePlugin,
    middleware: ({examplePlugin}) => (ctx, next) => {
      // And we can add new behavior here!

This pattern is used extensively in many of the plugins shipped with Fusion.js, such as adding injecting and modifying action payloads emitted by Redux.