在js代碼中掖鱼，除了同步代碼然走，就是異步代碼，一個(gè)js文件是從上往下執(zhí)行戏挡，遇到ajax芍瑞，點(diǎn)擊事件。settimeout等異步事件褐墅，不會(huì)立即執(zhí)行拆檬，會(huì)被放在相應(yīng)的web api模塊中，這些異步代碼什么時(shí)候執(zhí)行妥凳，等待主線程執(zhí)行完畢秩仆，才會(huì)執(zhí)行異步代碼。
異步代碼有執(zhí)行先后順序猾封，這個(gè)先后順序怎么執(zhí)行的?
異步代碼分為：宏任務(wù)澄耍，微任務(wù)。
宏任務(wù)：script(整體代碼), setTimeout, setInterval, setImmediate, I/O, UI rendering晌缘。
微任務(wù)：process.nextTick, Promises, Object.observe, MutationObserver
每一次事件循環(huán)齐莲，都會(huì)執(zhí)行一個(gè)宏任務(wù)，在這個(gè)宏任務(wù)中磷箕，可能有還存在宏任務(wù)选酗，微任務(wù)，那如何去執(zhí)行這些微任務(wù)岳枷，和宏任務(wù)芒填？記住，但你執(zhí)行一個(gè)宏任務(wù)空繁，首先要執(zhí)行所以的微任務(wù)殿衰，遇到里面的宏任務(wù)把推進(jìn)宏任務(wù)事件隊(duì)列，等待某一次事件循環(huán)來執(zhí)行這個(gè)宏任務(wù)盛泡。
事件循環(huán)作用：每次都是執(zhí)行一個(gè)宏任務(wù)闷祥，把這個(gè)宏任務(wù)里面的可能存在的多個(gè)微任務(wù)執(zhí)行完，看到一個(gè)宏任務(wù)傲诵，順便把推進(jìn)宏任務(wù)隊(duì)列 一直這樣反復(fù)的執(zhí)行
注解：宏任務(wù)都包括同步代碼凯砍，宏任務(wù)代碼，微任務(wù)代碼 主要這三部分代碼
同步代碼>微任務(wù)>宏任務(wù)的執(zhí)行順序
https://zhuanlan.zhihu.com/p/26229293
https://zhuanlan.zhihu.com/p/26238030
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