Refactor (public/src/client/infinitescroll.js:40): reduce complexity in onScroll()

[saisankhe]

What the file does
The file implements the client-side infinite scrolling behaviour in NodeBB. It keeps a track of the window scroll events, calculates how far the user has scrolled relative to the container and then triggers a callback to load more content.

The scope of the refactoring
Refactored was done only on the onScroll() function. No changes were made to the behaviours of the function. I created a few helper functions to keep everything clean and easy to read.

Qlty Smell Addressed
Function with high complexity (count = 11): onScroll in public/src/client/infinitescroll.js from line 40. The following was the output of the Qlty-reported issue
public/src/client/infinitescroll.js 40 Function with high complexity (count = 11): onScroll

REFACTORING
in onScroll, the high complexity (count = 11) reduced adaptability since the logic was condensed into one function. This made it harder to understand and modify the code.

I refactored onScroll() by adding helper functions that did one job each.
- skipScroll() was made to have all the early-exit conditions
- getMetrics() was created to compute currentScrollTop, viewportHeight and scrollPercent
- getDirection(currentScrollTop) was created to compute scroll direction in relation to previousScrollTop
- getTrigger(metrics) contained trigger rules for when to call the callback

The changes made improved adaptability. The multiple responsibilities condensed into the onScroll() function were converted to a function each for easier reading. An alternative I considered was to make changes in the existing function but I was running into the same problem of the responsibilities being condensed into one function.

VALIDATION
I ran NodBB locally, opened the site in the browser, created a few posts myself by creating an account to trigger the infinite scroll function, opened the Console from the DevTools, scrolled down the posts page to trigger the infinite scrolling. I inserted a console.log('Sai Sankhe') statement which was shown as belows:

qlty smells after changes