What Types Does Lodash _.isNumber Accept?

The Lodash _.isNumber method is a utility function used to verify whether a given value qualifies as a number. This guide outlines the specific types of numbers and representations accepted by _.isNumber, explores special numeric edge cases like NaN and Infinity, and highlights common non-number primitives that the function rejects.

Primitive Numbers

_.isNumber returns true for all standard JavaScript primitive numeric values. This includes:

• Integers: Positive, negative, and zero (e.g., 0, -0, 42, -100).
• Floating-Point Numbers: Decimals of any precision (e.g., 3.14, -0.001).
• Scientific Notation: Numbers represented using exponential notation (e.g., 1e5, 2.5e-3).
• Non-Decimal Literals: Binary (0b1010), octal (0o755), and hexadecimal (0xFF) literals, as JavaScript evaluates these as standard numeric primitives at runtime.

Number Objects

Unlike the native JavaScript typeof operator—which evaluates new Number(x) as an "object"—Lodash's _.isNumber correctly identifies Number object instances.

_.isNumber(new Number(42)); // true
typeof new Number(42);      // "object"

The function checks both whether the value has a typeof of "number" and whether the internal [[Class]] or Object.prototype.toString tag matches [object Number].

Special Numeric Values

JavaScript classifies several special symbolic values under the Number type according to the IEEE 754 standard. Consequently, _.isNumber returns true for:

• Infinity and -Infinity: Positive and negative infinite values resulting from calculations such as division by zero.
• NaN (Not a Number): Despite representing an invalid mathematical result, NaN technically belongs to the JavaScript number type.

If your use case requires excluding NaN or infinite numbers, Lodash provides _.isFinite, which validates that a value is a number while filtering out NaN, Infinity, and -Infinity.

Values Not Accepted by _.isNumber

To prevent unexpected behavior, _.isNumber returns false for values that might look like numbers or represent numbers in other formats:

• Numeric Strings: String representations of numbers such as '42', '3.14', or '0xFF' return false.
• BigInt: Large integer primitives (e.g., 42n or BigInt(42)) belong to the distinct bigint primitive type and return false.
• Other Non-Primitives and Nil Values: null, undefined, false, true, symbols, arrays, and standard objects all return false.