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Why does 0.1 + 0.2 not equal 0.3 exactly in JavaScript? What does Number.isInteger() check, and how is it different from typeof? What are some commonly used Math object methods? What is BigInt and how do you create one? Why does 9007199254740993n + 1 throw a TypeError, but 9007199254740993n + 1n does not? Why is Number.parseFloat('123abc') valid but Number('123abc') returns NaN? How do you correctly compare two floating-point numbers for 'close enough' equality? Why is BigInt generally the wrong tool for representing monetary amounts with decimals, like $19.99? How do bitwise operators like >>>, &, and | behave differently on BigInt versus Number?
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JavaScript › Numbers, Math & BigInt

What is Number.MAX_SAFE_INTEGER and what breaks beyond it?

Intermediate

Answer

It's 2^53 - 1 (9007199254740991), the largest integer JavaScript's double-precision Number type can represent without losing precision. Beyond it, integers can silently round to a nearby representable value, causing incorrect arithmetic and equality checks.
console.log(Number.MAX_SAFE_INTEGER);           // 9007199254740991
console.log(9007199254740992 === 9007199254740993); // true! precision lost
Real-world example A bug where large database IDs (like 64-bit Snowflake IDs) get silently corrupted when passed through JavaScript as regular numbers.

Common follow-ups: How does BigInt solve this precision problem?

ArrayBuffer TypedArrays & Binary Data
All 10 Numbers, Math & BigInt questions