JS为什么能够执行并发请求？

什么是消息队列？

什么是事件循环？

事件循环的基本流程

事件循环的例子

console.log('main1');

process.nextTick(function() {
    console.log('process.nextTick1');
});

setTimeout(function() {
    console.log('setTimeout');
    process.nextTick(function() {
        console.log('process.nextTick2');
    });
}, 0);

new Promise(function(resolve, reject) {
    console.log('promise');
    resolve();
}).then(function() {
    console.log('promise then');
});

console.log('main2');

• main1
• promise
• main2
• process.nextTick1
• promise then
• setTimeout
• process.nextTick2