Functions, Scope & Object Model

Functions as Values

Functions are first-class objects in JavaScript: they can be passed, returned, stored, decorated, bound, and composed. This is the foundation of callbacks, middleware, event handlers, array methods, and functional-style APIs.

A function value can move independently from its original name
Higher-order functions take or return functions
A function can carry a closure
Call signature and object identity are separate ideas
Detached methods can lose this

Higher-order function

function withTiming(fn) {
  return async (...args) => {
    const start = performance.now()
    try {
      return await fn(...args)
    } finally {
      console.log(`took ${performance.now() - start}ms`)
    }
  }
}

const timedFetchUser = withTiming(fetchUser)

The wrapper returns a new function that adds behavior around the original one.

Common function-as-value patterns
Pattern	Shape	Example
Callback	Pass function to run later	button.addEventListener('click', handler)
Mapper/filter	Pass transformation/predicate	items.map(fn), items.filter(fn)
Factory	Return configured function	createValidator(schema)
Decorator	Wrap another function	withRetry(fetcher)
Middleware	Chain functions around request	auth → handler → logger