MDN says for await...of has two use-cases:
The
for await...ofstatement creates a loop iterating over async iterable objects as well as on sync iterables,...
I was previously aware of the former: async iterables using Symbol.asyncIterator. But I am now interested in the latter: synchronous iterables.
The following code iterates over a synchronous iterable - an array of promises. It appears to block progess on the fulfilment of each promise.
async function asyncFunction() {
try {
const happy = new Promise((resolve)=>setTimeout(()=>resolve('happy'), 1000))
const sad = new Promise((_,reject)=>setTimeout(()=>reject('sad')))
const promises = [happy, sad]
for await(const item of promises) {
console.log(item)
}
} catch (err) {
console.log(`an error occurred:`, err)
}
}
asyncFunction() // "happy, an error occurred: sad" (printed in quick succession, after about 5 seconds)The behavior appears to be akin to awaiting each promise in-turn, per the logic shown below. Is this assertion correct?
async function asyncFunction() {
try {
const happy = new Promise((resolve)=>setTimeout(()=>resolve('happy'), 1000))
const sad = new Promise((_,reject)=>setTimeout(()=>reject('sad')))
const promises = [happy, sad]
for(let p of promises) {
const item = await p
console.log(item)
}
} catch (err) {
console.log(`an error occurred:`, err)
}
}
asyncFunction() // "happy, an error occurred: sad" (printed in quick succession, after about 5 seconds)I ask because this pattern of code has an implicit rejection wire-up pitfall that Promise.all and Promise.allSettled avoid, and it seems strange to me that this pattern would be explicitly supported by the language.
window.addEventListener('unhandledrejection', () => {
console.log('unhandled rejection; `sad` was not being awaited at the time it rejected')
})
async function asyncFunction() {
try {
const happy = new Promise((resolve)=>setTimeout(()=>resolve('success'), 1000))
const sad = new Promise((_,reject)=>setTimeout(()=>reject('failure')))
const promises = [happy, sad]
for(let p of promises) {
const item = await p
console.log(item)
}
} catch (err) {
console.log(`an error occurred:`, err)
}
}
asyncFunction() // "unhandled rejection; `sad` was not being awaited at the time it rejected" (after about zero seconds), and then "happy, an error occurred: sad" (printed in quick succession, after about 5 seconds)
