mine/node_modules/@hapi/hoek/lib/merge.js

'use strict';

const Assert = require('./assert');
const Clone = require('./clone');
const Utils = require('./utils');


const internals = {};


module.exports = internals.merge = function (target, source, options) {

    Assert(target && typeof target === 'object', 'Invalid target value: must be an object');
    Assert(source === null || source === undefined || typeof source === 'object', 'Invalid source value: must be null, undefined, or an object');

    if (!source) {
        return target;
    }

    options = Object.assign({ nullOverride: true, mergeArrays: true }, options);

    if (Array.isArray(source)) {
        Assert(Array.isArray(target), 'Cannot merge array onto an object');
        if (!options.mergeArrays) {
            target.length = 0;                                                          // Must not change target assignment
        }

        for (let i = 0; i < source.length; ++i) {
            target.push(Clone(source[i], { symbols: options.symbols }));
        }

        return target;
    }

    const keys = Utils.keys(source, options);
    for (let i = 0; i < keys.length; ++i) {
        const key = keys[i];
        if (key === '__proto__' ||
            !Object.prototype.propertyIsEnumerable.call(source, key)) {

            continue;
        }

        const value = source[key];
        if (value &&
            typeof value === 'object') {

            if (target[key] === value) {
                continue;                                           // Can occur for shallow merges
            }

            if (!target[key] ||
                typeof target[key] !== 'object' ||
                (Array.isArray(target[key]) !== Array.isArray(value)) ||
                value instanceof Date ||
                (Buffer && Buffer.isBuffer(value)) ||               // $lab:coverage:ignore$
                value instanceof RegExp) {

                target[key] = Clone(value, { symbols: options.symbols });
            }
            else {
                internals.merge(target[key], value, options);
            }
        }
        else {
            if (value !== null &&
                value !== undefined) {                              // Explicit to preserve empty strings

                target[key] = value;
            }
            else if (options.nullOverride) {
                target[key] = value;
            }
        }
    }

    return target;
};