promise问题 - CNode技术社区

怎么我的代码先打印‘download over’,后执行下载呢???

function downloadAsync(url, name) {
 return new Promise(function(resolve, reject) {
 request(url, function(err, response, body) {
 if (err) {
 reject(err);
 } else {
 fs.writeFileSync(path.resolve(process.cwd(), 'downloaded_html', name), body);
 console.log('got ' + url);
 resolve();
 }
 })
 })
}
var urls = ['url1', 'url2', 'url3']
var promises = urls.map(function(url, i) {
 var name = i + '.html'
 downloadAsync(url, name)
})
Promise.all(promises).then(function() {
 console.log('download over')
})

var promises = urls.map(function(url, i) {
 var name = i + '.html'
 return downloadAsync(url, name)
})

加个return就好了

老外也帮我回答了 http://stackoverflow.com/questions/36520082/why-my-code-print-download-over-first-and-then-start-to-download-html-pages

用箭头函数,可以省略 return

var promises = urls.map((url, i) => downloadAsync(url, i + '.html'))

Array.prototype.map 的并发是 Infinity, 要控制并发可以使用

• promise.map https://github.com/magicdawn/promise.map
• bluebird 提供的 Promise.map 支持一个 concurrency 参数,表示并发