Moderate Web APIs Question 121 of 229

What is Intersection Observer used for?

Frontend Master · Speak this in 60–90 seconds · Faridabad & Delhi NCR

IdeaWeb APIs

HowWhat happens inside

Why they askShows real use

It tells you when an element enters the viewport without scroll listeners.

Why interviewers care about Web APIs:

Web APIs questions separate

people who only read docs from people who shipped.

Keep it short, concrete,

and tied to Frontend work.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

Before you speak the answer, walk the interviewer through these steps:

1
It tells you when
an element enters the viewport without scroll listeners.
2
I use it for
lazy images, infinite lists, and analytics impressions.
3
The callback runs off
the main-thread scheduling of the browser, which is cheaper than measuring in scroll.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

Here's a short line you can speak, broken into clear beats:

Say this line

“I use it for lazy images, infinite lists, and analytics impressions.”

Break into beats

Iuseitforlazyimages

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

It tells you when an element enters the viewport without scroll listeners. I use it for lazy images, infinite lists, and analytics impressions.