Convert Object to JSON String - Solution & Explanation

Problem Overview: Given a JavaScript object containing values like numbers, strings, booleans, arrays, nested objects, and null, generate a valid JSON string representation without using JSON.stringify. The serializer must handle nested structures and produce correctly formatted JSON.

Approach 1: Built-in Serialization (Baseline) (O(n) time, O(n) space)

The simplest way to convert an object to JSON is calling JSON.stringify(obj). The runtime internally traverses the object graph, serializes primitives, and recursively processes arrays and nested objects. Every element or property is visited once, so the time complexity is O(n) where n is the total number of values in the structure. The resulting string requires O(n) space. This approach works in real applications but is disallowed in this problem because the goal is implementing the serializer logic manually.

Approach 2: Recursive Object Traversal (O(n) time, O(n) space)

The expected solution walks the structure recursively and builds the JSON string step by step. Check the value type first. If the value is null, return the literal "null". For numbers and booleans, convert them directly using string conversion. For strings, wrap the value in double quotes.

Arrays require iterating through each element, recursively serializing them, and joining the results with commas inside square brackets. Objects require iterating over their key-value pairs, converting the key to a quoted string, serializing the value recursively, and joining pairs with commas inside curly braces. Each recursive call processes one element of the structure, which makes the traversal linear.

The key insight: treat arrays and objects as containers and delegate serialization of their contents back to the same function. This recursive pattern mirrors how JSON structures are nested. Each value is processed exactly once, leading to O(n) time complexity. The output string and recursion stack together require O(n) space.

This pattern appears frequently in problems involving tree-like or nested data structures. Understanding recursive traversal helps with tasks like AST processing, deep cloning, and parsing. Related techniques appear in recursion, nested structure traversal, and string construction problems.

Recommended for interviews: The recursive traversal approach is what interviewers expect. Mentioning the built-in serializer shows awareness of practical JavaScript, but implementing the recursion demonstrates understanding of nested structures and algorithmic reasoning.