isFunction

The isFunction type guard checks if a value is a function.

Basic Usage

import { isFunction } from "cx/util";

isFunction(() => {}); // true
isFunction(function () {}); // true
isFunction(Math.max); // true
isFunction(Array.isArray); // true
isFunction(class Foo {}); // true (classes are functions)
isFunction(async () => {}); // true
isFunction("string"); // false
isFunction(42); // false
isFunction(null); // false
isFunction({}); // false

Type Narrowing

The function is a TypeScript type guard that narrows the type to Function.

import { isFunction } from "cx/util";

function invokeIfFunction(value: unknown): unknown {
  if (isFunction(value)) {
    // value is typed as 'Function' here
    return value();
  }
  return value;
}

Common Use Cases

Callback Handling

import { isFunction } from "cx/util";

interface Options {
  onSuccess?: () => void;
  onError?: (error: Error) => void;
}

function execute(options: Options): void {
  try {
    // ... do work
    if (isFunction(options.onSuccess)) {
      options.onSuccess();
    }
  } catch (error) {
    if (isFunction(options.onError)) {
      options.onError(error as Error);
    }
  }
}

Dynamic Property Resolution

import { isFunction } from "cx/util";

type ValueOrGetter<T> = T | (() => T);

function resolveValue<T>(valueOrGetter: ValueOrGetter<T>): T {
  return isFunction(valueOrGetter) ? valueOrGetter() : valueOrGetter;
}

resolveValue(42); // 42
resolveValue(() => 42); // 42

Event Handler Validation

import { isFunction } from "cx/util";

function addEventListener(
  element: HTMLElement,
  event: string,
  handler: unknown
): void {
  if (!isFunction(handler)) {
    throw new Error(`Handler for "${event}" must be a function`);
  }
  element.addEventListener(event, handler as EventListener);
}

Factory Pattern

import { isFunction } from "cx/util";

type Factory<T> = T | (() => T);

function createInstance<T>(factory: Factory<T>): T {
  return isFunction(factory) ? factory() : factory;
}

const config = createInstance({ name: "app" });
const dynamicConfig = createInstance(() => ({ name: "dynamic-app" }));

API

function isFunction(f: any): f is Function;

Returns: true if the value is a function (including arrow functions, methods, and classes).