Async JavaScript

Promises & Error Handling

A Promise represents an eventual fulfillment or rejection. Promise chains propagate returned values and thrown errors, letting async work be composed without nested callbacks.

A Promise has a settled outcome
Returning from then transforms the chain
Errors propagate until caught
Floating promises are invisible work
Promise errors are not synchronous try/catch unless awaited

Promise chain propagation

A chain is a pipeline of fulfillment and rejection states.

Propagating useful async errors

async function loadUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`)
    if (!res.ok) throw new Error(`HTTP ${res.status}`)
    return await res.json()
  } catch (err) {
    throw new Error(`Failed to load user ${id}`, { cause: err })
  }
}

Add context at the boundary, but preserve the original cause.