콜백 → Promise → async/await

// 콜백
fetchData(() => {
  processData(() => {
    displayData()
  })
})

// Promise 체인
fetchData().then(processData).then(displayData)

// async/await
async function main() {
  const data = await fetchData()
  const processed = await processData(data)
  await displayData(processed)
}

Promise는 모나드처럼 동작 — then 은 콜백이 Promise 를 돌려주면 풀어서 잇는다(flatMap). 값을 돌려주면 감싼다(map). 한 메서드가 둘 다 해서 Promise<Promise<T>> 는 만들어지지 않는다.1

Footnotes

  1. Promise.prototype.then() — MDN: “The value returned from then() is resolved in the same way as Promise.resolve(). This means thenable objects are supported, and if the return value is not a promise, it’s implicitly wrapped in a Promise and then resolved.” ↩

#javascript#monad#promise
`#536`