Parsing #2 — URL & Query String Parsing

URL query strings encode structured data in a flat key-value format. Parsing them into objects — and building them back — is a common interview question and everyday utility. This article implements both directions from scratch.

Prerequisites: Data #2 — Flatten & Unflatten Objects

1. Parse Query String → Object

2. Object → Query String

3. Parse Nested Query Keys (Bracket Notation)

Some APIs use user[name]=Naveen&user[role]=Engineer for nested data:

4. Parse URL — Extract Protocol, Host, Path, Query

5. Full URL Builder

Key Takeaways

• Always use encodeURIComponent() / decodeURIComponent() when handling query string values.
• Repeated keys are conventionally parsed into arrays.
• Bracket notation (user[name]) converts to nested objects using path splitting.
• For production, use URL and URLSearchParams APIs — these implementations show the internals.
• A URL has 6 main parts: protocol, host (hostname + port), pathname, search, hash.

Next: Parsing #3 — Building a Template Engine — replace {{ variables }} and add loops and conditionals.