Refactored file jsguide.js

[khaidao000]

Refactoring includes:

Using querySelector for simplicity where possible.
Utilizing Array.prototype.forEach.call to directly iterate over NodeList.
Utilizing const for variable declarations where possible.
Simplifying if conditions.
Grouping similar logic and segregating different logic with line breaks for readability.

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[vapier]

please break this up into multiple changes. there's good stuff in here, but squashing all of this rewrite into a single commit makes it difficult/impossible to reasonably review, and makes it hard to detect bugs or unintentional changes in behavior.