webpack/lib/util/cleverMerge.js

Packs CommonJs/AMD modules for the browser. Allows to split your codebase into multiple bundles, which can be loaded on demand. Support loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/

"use strict";

const mergeCache = new WeakMap();

/**
 * Merges two given objects and caches the result to avoid computation if same objects passed as arguments again.
 * @example
 * // performs cleverMerge(first, second), stores the result in WeakMap and returns result
 * cachedCleverMerge({a: 1}, {a: 2})
 * {a: 2}
 *  // when same arguments passed, gets the result from WeakMap and returns it.
 * cachedCleverMerge({a: 1}, {a: 2})
 * {a: 2}
 * @param {object} first first object
 * @param {object} second second object
 * @returns {object} merged object of first and second object
 */
const cachedCleverMerge = (first, second) => {
	let innerCache = mergeCache.get(first);
	if (innerCache === undefined) {
		innerCache = new WeakMap();
		mergeCache.set(first, innerCache);
	}
	const prevMerge = innerCache.get(second);
	if (prevMerge !== undefined) return prevMerge;
	const newMerge = cleverMerge(first, second);
	innerCache.set(second, newMerge);
	return newMerge;
};

/**
 * Merges two objects. Objects are not deeply merged.
 * TODO webpack 5: merge objects deeply clever.
 * Arrays might reference the old value with "..."
 * @param {object} first first object
 * @param {object} second second object
 * @returns {object} merged object of first and second object
 */
const cleverMerge = (first, second) => {
	const newObject = Object.assign({}, first);
	for (const key of Object.keys(second)) {
		if (!(key in newObject)) {
			newObject[key] = second[key];
			continue;
		}
		const secondValue = second[key];
		if (!Array.isArray(secondValue)) {
			newObject[key] = secondValue;
			continue;
		}
		const firstValue = newObject[key];
		if (Array.isArray(firstValue)) {
			const newArray = [];
			for (const item of secondValue) {
				if (item === "...") {
					for (const item of firstValue) {
						newArray.push(item);
					}
				} else {
					newArray.push(item);
				}
			}
			newObject[key] = newArray;
		} else {
			newObject[key] = secondValue;
		}
	}
	return newObject;
};

exports.cachedCleverMerge = cachedCleverMerge;
exports.cleverMerge = cleverMerge;