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script标签中defer和async的区别 #19

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conan1992 opened this issue Jun 16, 2020 · 0 comments

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conan1992 commented Jun 16, 2020 •

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默认情况下：脚本的下载和执行是按照文档顺序同步进行的，脚本下载和执行时，会阻塞文档的解析；当脚本下载和执行完毕时才会继续文档解析；

async

当script标签中有async，脚本加载和文档加载异步执行的，当脚本下载完会停止html解析，执行脚本，等脚本执行完再继续html解析；

defer

当script标签中有defer，脚本加载和文档加载异步执行的，当文档解析完毕后触发DOMContentLoaded 事件前执行；

async和defer同时存在时结果和async一样；

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