Execute an array of async tasks in sequence

Question

Given an array of async functions (each returning a Promise), execute them one after another — not concurrently — and collect the results.

Accepted Answer

// Using Array.reduce
function runSequential(tasks) {
  return tasks.reduce(
    (chain, task) => chain.then(results =>
      task().then(value => [...results, value])
    ),
    Promise.resolve([])
  )
}

// Using async/await (often cleaner in interviews)
async function runSequential(tasks) {
  const results = []
  for (const task of tasks) {
    results.push(await task())
  }
  return results
}

// Test
const tasks = [
  () => Promise.resolve(1),
  () => new Promise(res => setTimeout(() => res(2), 100)),
  () => Promise.resolve(3),
]
runSequential(tasks).then(console.log) // [1, 2, 3]

Execute an array of async tasks in sequence

Answer