source: trip-planner-front/node_modules/enhanced-resolve/lib/MainFieldPlugin.js

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

"use strict";

const path = require("path");
const DescriptionFileUtils = require("./DescriptionFileUtils");

/** @typedef {import("./Resolver")} Resolver */
/** @typedef {import("./Resolver").ResolveStepHook} ResolveStepHook */
/** @typedef {{name: string|Array<string>, forceRelative: boolean}} MainFieldOptions */

const alreadyTriedMainField = Symbol("alreadyTriedMainField");

module.exports = class MainFieldPlugin {
        /**
         * @param {string | ResolveStepHook} source source
         * @param {MainFieldOptions} options options
         * @param {string | ResolveStepHook} target target
         */
        constructor(source, options, target) {
                this.source = source;
                this.options = options;
                this.target = target;
        }

        /**
         * @param {Resolver} resolver the resolver
         * @returns {void}
         */
        apply(resolver) {
                const target = resolver.ensureHook(this.target);
                resolver
                        .getHook(this.source)
                        .tapAsync("MainFieldPlugin", (request, resolveContext, callback) => {
                                if (
                                        request.path !== request.descriptionFileRoot ||
                                        request[alreadyTriedMainField] === request.descriptionFilePath ||
                                        !request.descriptionFilePath
                                )
                                        return callback();
                                const filename = path.basename(request.descriptionFilePath);
                                let mainModule = DescriptionFileUtils.getField(
                                        request.descriptionFileData,
                                        this.options.name
                                );

                                if (
                                        !mainModule ||
                                        typeof mainModule !== "string" ||
                                        mainModule === "." ||
                                        mainModule === "./"
                                ) {
                                        return callback();
                                }
                                if (this.options.forceRelative && !/^\.\.?\//.test(mainModule))
                                        mainModule = "./" + mainModule;
                                const obj = {
                                        ...request,
                                        request: mainModule,
                                        module: false,
                                        directory: mainModule.endsWith("/"),
                                        [alreadyTriedMainField]: request.descriptionFilePath
                                };
                                return resolver.doResolve(
                                        target,
                                        obj,
                                        "use " +
                                                mainModule +
                                                " from " +
                                                this.options.name +
                                                " in " +
                                                filename,
                                        resolveContext,
                                        callback
                                );
                        });
        }
};