Frontend System Design in Action: Implementing Typeahead

In this issue, I want to show how I would apply the CCDAO framework in a frontend system design interview.

A few months ago, I introduced the framework and explored each layer in depth. This time — and in the next issue — I want to walk through how it works in practice.

Let’s start with a relatively common interview question: Design a typeahead search box.

Photo by Jackson Sophat on Unsplash

It sounds manageable. You picture an input, a list of suggestions, maybe a quick API call.

But after a minute of thinking, questions start to surface.

• What if the results are personalised?

• What if the user is offline?

• How should you handle loading states, debouncing, and keyboard navigation?

Simple features like this often hide more complexity than they show.

In moments like that, I rely on a framework — not because it gives me the answers, but because it helps me organise my thinking.

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The CCDAO Framework

I’ve discussed the CCDAO framework before https://juntao.substack.com/p/navigating-frontend-system-design and https://juntao.substack.com/p/deeper-dive-into-the-ccdao-framework, but if you’re new: it’s a structure I use to reason through frontend system design problems.

C → Collect Information
C → Component Structure
D → Data Modelling
A → API Design
O → Optimisation Strategies

Each layer leads naturally into the next. I don’t treat it as a checklist — more like a lens I switch between as needed.

You may have seen this sketch in the video version of this issue:

Here’s how I would apply the framework when designing a typeahead search box.

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1. Collect Information

Before designing anything, I try to understand what I’m actually solving. That’s just as important as technical knowledge during an interview.

Use the time to gather context. Don’t just focus on the happy path — edge cases, error handling, and non-functional requirements matter just as much (if not more).

Questions I typically ask:

• What’s the scale? Are we searching a local list or querying a remote index of millions?

• Are the results personalised per user?

• What platforms are supported — desktop, mobile, both?

• Do we have accessibility or internationalisation requirements?

• Can we cache results, or are they fetched fresh each time?

• Should we support pagination or infinite scrolling?

• What does a result look like — just text, or images, tags, metadata?

• Do we need keyboard navigation? Should suggestions appear on focus or only after typing?

These aren’t just implementation details — they influence the design from the ground up.