/**
 * Should return new value basing on property name and value. Optionally may return exported REMOVE object to filter out
 * property from the object.
 *
 * @callback MapValuesFn
 * @param {*} value - property value
 * @param {string} key - property name
 * @example function fn(value, key) { return value * 5; } // all properties will be multiplied
 * @example function fn(value, key) {
 *   if (key === "name") {
 *     return REMOVE;
 *   }
 *   return value.toUpperCase();
 * }
 * // will upper case all properties and filter out `name` property
 * @returns {*}
 */
const REMOVE = typeof Symbol !== "undefined" ? Symbol("remove") : {};
type MapValuesFn<S, R> = (value: S[keyof S], key: keyof S) => R;

/**
 * Value that denotes that property should be removed
 *
 * @type {Object}
 */

/**
 * Iterates through object properties returning object with same properties but modified values. Optionally some
 * properties may be filtered out on returned object.
 *
 * @param {Object|Array} source - source object
 * @param {MapValuesFn} fn - map function callback that will return new value of a property
 * @example mapValues({ a: 1, b: 2 }, x => x * 2) // will return { a: 1, b: 4 }
 * @example mapValues({ a: 1, b: 2 }, () => REMOVE) // will return {}
 * @returns {Object|Array}
 */
const mapValues = <SourceObject extends object, PossibleReturnValues>(
    source: SourceObject, fn: MapValuesFn<SourceObject, PossibleReturnValues>,
): { [P in keyof SourceObject]: PossibleReturnValues } => {
    const keys = Object.keys(source) as (keyof SourceObject)[];

    // @ts-expect-error TypeScript doesn't work well with this type with reduce
    // @TODO check with ts 5+ or something
    const def: { [P in keyof SourceObject]: PossibleReturnValues } = Array.isArray(source) ? [] : {};

    return keys.reduce((result, key) => {
        const value = fn(source[key], key);
        if (value !== REMOVE) {
            result[key] = value; // eslint-disable-line no-param-reassign
        }
        return result;
    }, def);
};
mapValues.REMOVE = REMOVE;

export { mapValues };