isNegative(): test whether a number is below zero

isNegative(x) returns true when x is less than zero. It makes sign-sensitive formulas and validation rules easy to read.

Typical uses

Reject negative lengths, determine direction, or branch on a financial loss. isNegative(-3) is true; zero is neither negative nor positive.

Related checks

Use isPositive() for values above zero and isZero() for the remaining case. sign() returns the sign itself, while abs() removes it. Check isNumeric() first for untrusted input.

Boundary care

Very small floating-point values can be negative because of rounding. Use a tolerance when the real-world rule is “effectively zero”.