isFunction

Checks whether a value is a function.

This helper never throws and never mutates input. Use it when you need to confirm callable values before running logic that expects a function.

Signature

function isFunction(value: unknown): value is Function

Name	Data type	Description
value	unknown	The value to check. Must be a function (per JavaScript's typeof semantics), including class constructors, async functions, generator functions, and bound functions.

Returns

A boolean:

• true: If the value is a function (including classes, async, generator, and built-in functions).
• false: Otherwise.

Examples

// Standard functions
isFunction(function () {})        // true
isFunction(() => {})              // true

// Async functions
isFunction(async function () {})  // true
isFunction(async () => {})        // true

// Generator functions
isFunction(function* () {})       // true

// Class constructors
class MyClass {}
isFunction(MyClass)               // true

// Bound functions
const fn = () => {};
const bound = fn.bind(null);
isFunction(bound)                 // true

// Built-in functions
isFunction(Math.max)              // true
isFunction(Promise.resolve)       // true

// Non-functions
isFunction(123)                   // false
isFunction('abc')                 // false
isFunction({})                    // false
isFunction(null)                  // false
isFunction(undefined)             // false
isFunction([])                    // false
isFunction(Symbol())              // false

Notes

• Classes are technically functions in JavaScript (typeof class X {} === 'function'), so they return true.
• Async functions, generator functions, and bound functions are all considered functions by this helper.
• This helper does not invoke the value; it only checks type.
• Use normalizeFunction if you need to wrap or coerce values into callable functions.
• Use validateFunction if you need a Result<T> instead of a boolean.

Next up

isJSON