Callbacks

A callback is simply a function you pass as an argument, expecting it to be called later — either immediately (synchronous) or at some point in the future (asynchronous).

Simple examples

// Async callback — runs after 1 second
setTimeout(() => {
  console.log('fired!')
}, 1000)

// Sync callback — runs immediately for each element
const doubled = [1, 2, 3].map((n) => n * 2)

// Event callback — runs when the event occurs
button.addEventListener('click', function handleClick() {
  console.log('clicked')
})

Callback Hell (Pyramid of Doom)

When you chain multiple async operations that each depend on the previous result, you nest callbacks inside callbacks:

getUser(id, (user) => {
  getOrders(user.id, (orders) => {
    getOrderDetails(orders[0].id, (details) => {
      getItems(details.itemId, (items) => {
        // four levels deep and growing…
      })
    })
  })
})

This gets hard to read, hard to debug, and hard to handle errors (you'd need a try/catch or error callback at every level).