/**
 * A filter function used to determine which properties to copy.
 * @callback FilterFn
 * @param {any} key
 * @param {any} value
 * @returns {boolean} returns `true` for properties to be copied.
 */
/**
 *   Extends an object with properties from one or more source objects.
 *   @param {Object} dest - The object to be extended.
 *   @param {Object|Object[]} sources - The source object(s) from which to extend the `dest` object.
 *   @param {function=} filter - An optional function to filter which properties to extend. It should take two arguments (`key` and `value`) and return `true` if the property should be extended, `false` otherwise.
 *   @returns {Object} - The extended `dest` object.
 * @example
 * var dest = { a: 1 };
 * var src1 = { b: 2 };
 * var src2 = { c: 3 };
 * extend(dest, src1, src2);
 * // dest => { a: 1, b: 2, c: 3 }
 *
 * @example
 * var dest = { a: 1 };
 * var src1 = { b: 2 };
 * var src2 = { c: 3 };
 * var filter = function(key, value) {
 *   return key !== 'b.js';
 * };
 * extend(dest, [src1, src2], filter);
 * // dest => { a: 1, c: 3 }
 */
export function extend(dest: any, sources: any | any[], filter?: Function | undefined): any;
export default extend;
/**
 * A filter function used to determine which properties to copy.
 */
export type FilterFn = (key: any, value: any) => boolean;