async reduce of array

index.js

async function reduce(arr, fn) {
    let tasks = arr.map(item => ({ item })),
        reqs = []
    while (true) {
        if (tasks.length === 1) {
            return tasks[0].item
        }
        const toRun = tasks.filter(task => !task.started)
        for (let i = 0; i + 1 < toRun.length; i += 2) {
            const [a, b] = toRun.slice(i, i + 2)
            a.started = b.started = true
            const r = fn(a.item, b.item).then(item => {
                tasks = tasks.filter(task => task !== a && task !== b).concat({ item, a, b })
                reqs = reqs.filter(req => req !== r)
            })
            reqs.push(r)
        }
        await Promise.race(reqs)
    }
}

function sleep(time) {
    return new Promise(resolve => setTimeout(resolve, time))
}

async function it() {
    const arr = Array(Math.floor(Math.random() * 5 + 1)).fill(0).map((_) => Math.random())
    const ret = await reduce(arr, async function (a, b) {    
        await sleep(Math.random() * 2000)
        console.log(a, b)
        return a + b
    })
    console.log('assert', ret, '===', arr.reduce((s, a) => s + a, 0))
}

it()