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5. Full tutorial step by step
(yet to code)
Basically, you need to create a global counter, Increment it in the load event listener of each image, and fires a callback when the counter reaches the number of images.
6. Lazy loading
Apart from preloading, there is lazy loading. Preloading is for eliminating user's wait time when viewing the webpage, lazy loading is for reducing user's wait time before viewing the content.
A resource shared by Morpheus about lazy loading :
7. Why mark own answer best?
When I see best answer marked, I'll just skip the Q&A. That's why I won't know your question is not solved if Morpheus didn't tag me.
Is your behavior for some sololearn badge? If so, please don't mind these virtual achievements, which you cannot take away from Sololearn.
8. Psycology tricks of tech app
Here is a reading for you: