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Jun 27, 2025 · Fundamentals

Why Does num === num - 1 Sometimes Return True in JavaScript?

This article explains the special cases where the JavaScript expression num === num - 1 evaluates to true, covering Infinity, numbers beyond the safe integer range, how to detect unsafe values, and practical solutions using Number.isSafeInteger, BigInt, bignumber.js, and a custom big‑integer addition implementation.