Number Edge Cases (NaN, Infinity, Floating Point)

                Number.isNaN(x) / Number.isFinite(x)
              

Purpose

JavaScript has one numeric type, a 64-bit IEEE 754 floating-point number, used for both integers and decimals. That single representation produces a few well-known, predictable surprises.

Examples

console.log(0.1 + 0.2); // 0.30000000000000004
console.log(10 / 0); // Infinity
console.log(-10 / 0); // -Infinity
console.log(0 / 0); // NaN
console.log(typeof NaN); // "number"

Behavior notes

• Most decimal fractions have no exact binary floating-point representation, so arithmetic on them accumulates tiny rounding error. This isn’t specific to JavaScript — it’s true of floating point in nearly every language.
• Infinity and -Infinity result from dividing by zero (or overflowing the representable range) rather than throwing an error.
• NaN (“Not a Number”) results from an invalid numeric operation (0 / 0, Number("abc")). NaN is the only JavaScript value that is never equal to itself — NaN === NaN is false.

Common mistakes

• Comparing floating-point results for exact equality (0.1 + 0.2 === 0.3 is false). Compare within a small tolerance instead, or work in integers (cents instead of dollars) when exactness matters.
• Checking for NaN with === NaN or == NaN — both always evaluate to false. Use Number.isNaN(x).
• Confusing the global isNaN() with Number.isNaN() — the global version coerces its argument first (isNaN("abc") is true), while Number.isNaN("abc") is false because a string is never NaN itself, only an invalid conversion result.

Runtime notes

The Number type and its edge cases are defined by ECMAScript and behave identically across engines.