promises.reduce code example
Example 1: javascript reduce promises
const sum = await [
Promise.resolve(1),
Promise.resolve(1),
Promise.resolve(1)
].reduce(async (previousPromise, itemPromise) => {
const sum = await previousPromise;
const item = await itemPromise;
return sum + item;
}, Promise.resolve(0))
Example 2: es6 reduce return promise
async function slowCounter () {
const ret = await ([100, 200, 300, 400, 500]).reduce(
async (promise, wait, idx) => {
return promise.then(async last => {
const ret = last + wait
console.log(`${idx}: waiting ${wait}ms to return ${ret}`)
await new Promise((res, rej) => setTimeout(res, wait))
return ret
})
}, Promise.resolve(0))
console.log(ret)
}
slowCounter ()