Build a Query String Parser and Builder for Environments Without Native URLSearchParams

Technical Interview Challenge: The "Legacy-Link" Query Parser

Context

Before the modern URL and URLSearchParams APIs were standardized, frontend engineers had to manually manipulate the "Search" portion of a URL. Even today, when building high-performance micro-utilities or supporting highly restricted legacy environments (like older embedded browsers or specific enterprise runtimes), you cannot always rely on native browser APIs.

The challenge is to build a robust Data Structure that can handle the "Multi-Value" nature of query parameters—where a single key can represent multiple values (e.g., ?filter=red&filter=blue)—while maintaining a clean, predictable interface for modification and serialization.

Problem Statement

Implement a class MyURLSearchParams that mimics the standard browser API for parsing and manipulating URL query strings. Your implementation must handle encoding/decoding, multiple values per key, and state synchronization.

Requirements:

1. Stateful Storage: Store parameters in a way that preserves multiple values for a single key and maintains the order of insertion.

2. Core Methods:

   • constructor(init): Accepts a query string (optionally starting with ?).

   • get(key) / getAll(key): Retrieves the first value or all values associated with a key.

   • set(key, value): Replaces all existing values for a key with a single new value.

   • append(key, value): Adds a new value to a key without removing existing ones.

   • delete(key): Removes the key and all its associated values entirely.

3. Utility Methods:

   • has(key): Returns a boolean indicating if a key exists.

   • toString(): Serializes the current state back into a URL-encoded query string.

4. Edge Case Handling: Handle empty strings, malformed pairs (like key=), and ensure values are properly decoded/encoded.

Example Use Cases

Example 1: Parsing and Retrieval

JavaScript

Example 2: Modification and State

JavaScript

Example 3: Deletion

JavaScript

Interview Evaluation Criteria

• Data Structure Choice: Will you use a plain Object, a Map, or a Map<string, string[]>? How does your choice impact the toString() order and the append() logic?

• Encoding/Decoding: Do you correctly use decodeURIComponent during parsing and encodeURIComponent during serialization to handle special characters (like spaces, &, or =)?

• Order Preservation: When calling toString(), are the parameters returned in the order they were originally parsed or appended?

• Parsing Logic: How do you handle edge cases like ?key (no equals sign) or && (empty segments)?