JavaScript optional function helper

optional.js

/**
 * Copy properties from source to target.
 * @param target
 * @param source
 */
function copyProps(target, source) {
    const properties = Object.getOwnPropertyNames(source);

    properties
        .concat(Object.getOwnPropertySymbols(source))
        .filter((key) => !key.match(/^(?:constructor|prototype|arguments|caller|name|bind|call|apply|toString|length)$/))
        .forEach((key) => {
            const props = Object.getOwnPropertyDescriptor(source, key);
            Object.defineProperty(target, key, props);
        });
    
    return target;
}

/**
 * The optional function accepts any argument and allows you to access properties or call methods on that object.
 * If the given object is null, properties and methods will return null instead of causing an error.
 *
 * @example
 *   const source = { bar: 'bar' };
 *   optional(source).foo; // return null
 *   optional(source, 'baz').foo; // return 'baz'
 *   optional(source, null, true).foo(); // return null
 *   optional(source, 'baz', true).foo(); // return 'baz'
 *   optional(source, (src) => src.bar).foo; // return 'bar'
 *   optional(source, (src) => src.bar, true).foo(); // return 'bar'
 *
 * @param source
 * @param callback
 * @param defaultValue
 * @return {(function(): null)|{value}|null|*}
 */
function optional(source, defaultValue = null, callback = null) {
    const result = copyProps({}, source);

    return new Proxy(result, {
        get(target, prop) {
            if (target[prop] === undefined) {
                if (callback === true) {
                    if (typeof defaultValue === 'function') {
                        return () => defaultValue(source);
                    }

                    return () => defaultValue;
                } else if (typeof defaultValue === 'function') {
                    return defaultValue(source);
                }

                return defaultValue;
            }

            return target[prop];
        },
    });
}