瀏覽器的Event Loop

1. 微任務隊列的任務將在當前輪次的事件輪詢中被推入微任務隊列

2. 而macrotask隊列中的任務必須等待下一輪時事件循環

   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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