Functions, Scope & Object Model

Object Composition

Composition builds behavior by combining smaller capabilities rather than relying on deep inheritance. JavaScript makes composition natural because objects and functions are flexible runtime values.

Composition favors small capabilities
Decorators wrap behavior without changing the original
Dependency injection is composition
Mixins are composition with sharp edges
Composition is easier to test when boundaries are explicit

Composition over deep inheritance

Each wrapper adds one behavior around a smaller core.

Composed service wrapper

function withRetry(service, retries = 3) {
  return {
    async save(data) {
      let lastError
      for (let attempt = 0; attempt < retries; attempt++) {
        try {
          return await service.save(data)
        } catch (err) {
          lastError = err
        }
      }
      throw lastError
    },
  }
}

The wrapper composes retry behavior without changing the original service.

Reuse strategies
Strategy	Strength	Weakness
Class inheritance	Shared API and polymorphism	Can become rigid/deep
Composition	Flexible behavior assembly	Needs clear boundaries
Decorator	Wrap behavior locally	Too many wrappers can obscure flow
Mixin	Reuse method groups	Name collisions and hidden assumptions