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Add Large Integers as Strings

By FrontendAtlas Team · Updated Jan 31, 2026

Add two non-negative integers represented as strings and return the sum as a string. You may not convert the entire string to a number or use BigInt — simulate grade-school addition with carry.

What you’ll build / What this tests

This premium javascript coding focuses on Add Large Integers as Strings. You’ll apply strings and numbers thinking with intermediate level constraints. The prompt emphasizes Add two non-negative integers represented as strings and return the sum as a string. You may….

Learning goals

Translate the prompt into a clear javascript API signature and return shape.
Apply strings, numbers, math techniques to implement add large integers as strings.
Handle intermediate edge cases without sacrificing readability.
Reason about time/space complexity and trade-offs in javascript.

Key decisions to discuss

Define the exact input/output contract before coding.
Prioritize predictable edge-case handling over micro-optimizations.

Evaluation rubric

Correctness: covers required behaviors and edge cases.
Clarity: readable structure and predictable control flow.
Complexity: avoids unnecessary work for large inputs.
API discipline: no mutation of inputs; returns expected shape.
Testability: solution is easy to unit test.

Constraints / Requirements

Handle empty strings and mixed casing without errors.
Handle empty or missing inputs without throwing errors.
Keep runtime close to linear time where possible.
Prefer a pure function: no side effects beyond the return value.

Mini snippet (usage only)

// Example usage
const a = /* add large integers as strings input */;
const b = /* config */;
const result = addStrings(a, b);
console.log(result);

// Edge case check
const empty = a && b ?? null;
const fallback = addStrings(a, b);
console.log(fallback);

// Expected: describe output shape, not the implementation
// (no solution code in preview)

Common pitfalls

Mutating inputs instead of returning a new value.
Skipping edge cases like empty input, duplicates, or nulls.
Overlooking time complexity for large inputs.