高效请求并发控制

function concurrencyRequest (urls, limit) {
return new Promise(resolve => {
if(urls.length === 0) {
resolve([])
return
}
let nextIndex = 0
const result = []
let count = 0

    async function _request() {
        const idx = nextIndex
        nextIndex++
        try {
        	// 这个地方后边如果是同步任务的话，resolve的时候会多输出limit  - 1个结果。原因并发额外的递归任完成的时候，实际会执行urls.length +( limit - 1) 次result[idx]  = xxx, 而异步任务是微任务，会等执行栈结束才最后执行 result[idx]  = xxx
            result[idx] = await fetch(`https://jsonplaceholder.typicode.com/todos/${idx}`)
        } catch(e) {
            result[idx] = e
        }
       
        if(nextIndex < urls.length) {
            _request()
        }
        count++
        if(count >= urls.length) {
            resolve(result)
            return
        }

    }
    for(let i = 0; i < Math.min(urls.length, limit); i++) {
        _request()
    }
})

}
concurrencyRequest(new Array(20).fill(1), limit).then(res => {
console.log(res);
})