Why async/await with forEach is a bug

Question

Explain why `await arr.forEach(async ...)` does not wait, and show correct sequential and parallel alternatives.

Accepted Answer

// Pitfall:
// forEach ignores returned promises, so outer await does nothing useful.
async function wrong(items) {
  await items.forEach(async (item) => {
    await save(item)
  })
  console.log('done') // runs before saves finish
}

// Sequential (ordered):
async function sequential(items) {
  for (const item of items) {
    await save(item)
  }
}

// Parallel (faster, unordered completion):
async function parallel(items) {
  await Promise.all(items.map((item) => save(item)))
}

// Rule: use for...of for sequential work, Promise.all(map) for parallel work.

Why async/await with forEach is a bug

Answer