Edge Cases

Empty Inputs

No data at all: empty array, empty string, empty request body. Watch for divide-by-zero, index errors, or assumptions like arr[0]. Always define expected output for empty input.

Single Element

Exactly one item instead of many. Common issues include off-by-one errors, incorrect initialization, and loops that never or barely run.

Duplicate Data

Repeated values where uniqueness might be assumed. Can cause double counting, incorrect logic, or infinite loops if not handled explicitly.

Invalid or Malformed Data

Bad formats, wrong types, corrupted payloads, or unexpected structure. Risks include crashes, silent corruption, and security issues.

Extreme Values

Very large or very small numbers, very old or future dates. Can cause overflow, precision loss, performance degradation, or timeouts.

Timezone Issues

Mixing UTC and local time or crossing DST boundaries. Common bugs include off-by-hours/day errors and incorrect ordering or expiration logic.

Missing Optional Fields

Fields that are allowed to be absent. Can cause KeyErrors or undefined behavior if defaults are not handled correctly.

Null or Undefined Values

Explicit null/None/undefined references. Common failure is null pointer exceptions. Always guard before dereferencing.

How to Explain Edge Cases in Interviews

I reason about empty, minimal, duplicate, malformed, extreme, time-related, and null inputs, and define expected behavior before implementation.