Dependency injection is where an object supplies dependencies for another object. In this example project some CommonJS modules, such as unit tests, are able to override require() dependencies of other modules in the project.

Every module imports other modules using the project root index.js file which is the central hub for all required modules.

The root index.js file exports an object with lazy loaded require statements. And the results of these statements can be overriden using arguments to Object.assign().

For example,

• thirdparty/index.js is a module that exports { debug }
• src/index.js is a module that exports { Widget }
• In a unit test, we want to override debug with a mock.
• In the unit test, before importing the Widget, we override debug like so:

const sinon = require('sinon')
const { debug: createDebug } = require('../index').thirdparty({ debug: sinon.stub() })
const { Widget } = require('../index').app()

Because the root index.js module is cached by the require.cache, its returned keys such as debug are now cached with the new sinon.stub() object instead of the original debug module. Allowing the unit test to check calls on debug().

The syntax that allows this, in the root index.js file, looks like this -

module.exports = {
    app: (inject) => Object.assign(require('./src'), inject),
    thirdparty: (inject) => Object.assign(require('./thirdparty'), inject),
}

Should a more involved hierarchy of dependencies be needed, the same pattern can be extended to do a deep Object.assign instead of the shallow example.