isNegativeNumber

Checks whether a value is a negative number.

This helper never throws and never mutates input. Returns true only if the value is a finite number primitive whose value is strictly less than zero.

Fractional values are allowed. Strings, objects, arrays, booleans, and boxed numbers are rejected.

Signature

function isNegativeNumber(value: unknown): value is number

Parameters

Name	Type	Description
value	unknown	The value to check. Must be a finite number less than zero (0).

Returns true if the value is a negative number, otherwise false.

Examples

isNegativeNumber(-1);       // → true
isNegativeNumber(-3.14);    // → true
isNegativeNumber(0);        // → false
isNegativeNumber(5);        // → false
isNegativeNumber("5");      // → false
isNegativeNumber(NaN);      // → false
isNegativeNumber(-Infinity);// → false

Notes

• Zero is not considered negative.
• Fractional numbers less than zero are allowed.
• Only number primitives are accepted; boxed numbers (new Number(-5)) are rejected.
• Never throws. Always returns a boolean.
• Use validateNegativeNumber if you need a structured ValidationResult.