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What does Number.isInteger() check, and how is it different from typeof? What are some commonly used Math object methods? What is Number.MAX_SAFE_INTEGER and what breaks beyond it? 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

Why does 0.1 + 0.2 not equal 0.3 exactly in JavaScript?

Beginner

Answer

JavaScript numbers use the IEEE 754 double-precision floating-point format, which can't represent most decimal fractions exactly in binary — this is a property of floating-point math shared by nearly all programming languages, not a JavaScript-specific bug.
console.log(0.1 + 0.2);            // 0.30000000000000004
console.log(0.1 + 0.2 === 0.3);    // false
Real-world example Avoiding == comparisons on computed floating-point totals in a shopping cart or financial calculation.

Common follow-ups: How do you correctly compare two floating-point numbers for 'close enough' equality?

Types & Coercion
All 10 Numbers, Math & BigInt questions