/**
 * Copyright (c) 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 */
/* eslint-disable header/header */

/**
 * Constructs an enumeration with keys equal to their value.
 *
 * For example:
 *
 *   var COLORS = keyMirror({blue: null, red: null});
 *   var myColor = COLORS.blue;
 *   var isColorValid = !!COLORS[myColor];
 *
 * The last line could not be performed if the values of the generated enum were
 * not equal to their keys.
 *
 *   Input:  {key1: val1, key2: val2}
 *   Output: {key1: key1, key2: key2}
 *
 * @param {object} obj
 * @return {object}
 */
export default function keyMirror<T extends {}>(obj: T): { [K in keyof T]: K } {
    if (!(obj instanceof Object && !Array.isArray(obj))) {
        throw new Error('keyMirror(...): Argument must be an object.');
    }

    const ret: any = {};
    for (const key in obj) {
        if (!obj.hasOwnProperty(key)) {
            continue;
        }

        ret[key] = key;
    }
    return ret;
}