浏览器的Event Loop

1. 微任务队列的任务将在当前轮次的事件轮询中被推入微任务队列

2. 而macrotask队列中的任务必须等待下一轮时事件循环

   macrotasks: setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O, UI rendering
   microtasks: process.nextTick, Promises, Object.observe, MutationObserver

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when a task (in macrotask queue) is running,new events may be registered.So new tasks may be created.Below are two new created tasks:
promiseA.then()'s callback is a task
promiseA is resolved/rejected: the task will be pushed into microtask queue in current round of event loop.
promiseA is pending: the task will be pushed into microtask queue in the future round of event loop(may be next round)
setTimeout(callback,n)'s callback is a task,and will be pushed into macrotask queue,even n is 0;
task in microtask queue will be run in the current round,while task in macrotask queue has to wait for next round of event loop.
we all know callback of "click","scroll","ajax","setTimeout"... are tasks,however we should also remember js codes as a whole in script tag is a task(a macrotask) too.

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