Basic concepts
<code>javascript是一门解释型的语言，浏览器充当解释器。 js执行引擎并不是一行一行的执行，而是一段一段的分析执行。 </code>
Delay Script
The defer attribute is defined in HTML4.0.1. Its purpose is to indicate that the script will not affect the structure of the page when executed. In other words, the script will be delayed until the entire page has been parsed before execution. Therefore, setting the defer attribute in the
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