@shko.online/dataverse-odata/src/getFetchXmlFromParser.ts

This package will help parse OData strings (only the Microsoft Dataverse subset). It can be used as a validator, or you can build some javascript library which consumes the output of this library.

import type { ODataQuery, ODataFetch } from './OData.types';
import { atMostOnce } from './validators/atMostOnce';
import { differentFromEmptyString } from './validators/differentFromEmptyString';

const option = 'fetchXml';

/**
 * Parses the {@link ODataFetch.fetchXml fetchXml} query
 * @returns Returns `false` when the parse has an error
 */
export const getFetchXmlFromParser = (parser: URLSearchParams, result: ODataQuery): boolean => {
    const value = parser.getAll(option);
    if (value.length === 0) {
        return true;
    }
    if (!atMostOnce(option, value, result) || !differentFromEmptyString(value, result)) {
        return false;
    }

    const fetchXml = value[0];
    const serializer = new DOMParser();
    const fetchXmlDocument = serializer.parseFromString(fetchXml, 'text/xml');
    if (fetchXmlDocument.documentElement.tagName === 'parsererror') {
        result.error = {
            code: '0x80040201',
            message: 'Invalid XML.',
        };
        return false;
    }
    const entity = fetchXmlDocument
        .evaluate('fetch/entity', fetchXmlDocument, null, XPathResult.ANY_TYPE, null)
        .iterateNext() as Element;
    if (fetchXmlDocument.documentElement.children.length != 1 || !entity || !entity.getAttribute('name')) {
        result.error = {
            code: '0x80041102',
            message: 'Entity Name was not specified in FetchXml String.',
        };
        return false;
    }
    const invalidAttribute = fetchXmlDocument
        .evaluate(
            'fetch/entity/*[not(self::filter or self::order or self::link-entity or self::attribute or self::all-attributes or self::no-attrs)]',
            fetchXmlDocument,
            null,
            XPathResult.ANY_TYPE,
            null,
        )
        .iterateNext() as Element;
    if (invalidAttribute) {
        result.error = {
            code: '0x8004111c',
            message: `Invalid Child Node, valid nodes are filter, order, link-entity, attribute, all-attributes, no-attrs. NodeName = ${invalidAttribute.tagName} NodeXml = ${invalidAttribute.outerHTML}`,
        };
        return false;
    }
    result.fetchXml = fetchXmlDocument;

    return true;
};