/**
 * Determine if an item is a function
 *
 * NOTE: While this isn't the fastest performing test in every browser, it is the faster when averaged across the browsers we care about.
 *
 * @param x - The item to test
 * @returns The result
 *
 * @example
 * isFunction(() => {}); // Returns: true
 * isFunction('foo');    // Returns: false
 */
export declare const isFunction: (x: any) => x is Function;