Promise.all 同时执行多个任务

promise-all.js

// http://stackoverflow.com/questions/30362733/handling-errors-in-promise-all
// https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
(function() {

    function randomNum(min, max) {
        return Math.floor(Math.random() * (max - min) + min); 
    }

    function upload(file) {
        return new Promise(function(resolve, reject) {
            setTimeout(function() {
                if(randomNum(1000, 2000) % 2 === 0) {
                    resolve(file + ' - success')
                } else {
                    reject(file + ' - fail');
                }
            }, randomNum(1000, 2000))
        })
    }

    function main() {
        var tasks = [];
        for(var i = 0; i < 5; i++) {
            tasks.push(
                upload(i).catch(function(err) {
                    return err
                })
            )
        }
        return Promise.all(tasks)
    }

    main().then(function(value) {
        console.log(value)
    })
    .catch(function(err) {
        console.log(err)
    })

})()