@sector-labs/seo-slip/src/checkers/canonicalChecker.js

Catches SEO regressions by sampling and checking a website against a set of rules

const xpath = require('xpath-html');

const { isHtmlDocument, tryGetContentByXPath } = require('../html');

const { newMessage, newEmptyItemResult } = require('../reporting');

module.exports = (canonicalRules) => {
    const name = 'canonicalChecker';

    const getCanonicalUrl = (responseBody) => {
        const body = xpath.fromPageSource(responseBody);
        const hrefAttributeValue = tryGetContentByXPath(
            body,
            `//link[@rel="canonical"]/@href`
        ).join(' ');
        return hrefAttributeValue;
    };

    return {
        analysis: (queueItem, responseBody, response) => {
            const isHtmlDoc = isHtmlDocument(responseBody, response);
            return {
                url: queueItem.url,
                isHtmlDocument: isHtmlDoc,
                canonicalUrl: isHtmlDoc ? getCanonicalUrl(responseBody) : '',
            };
        },
        report: (analysis) => {
            return {
                canonicalUrl: analysis.canonicalUrl,
            };
        },
        check: (analysis) => {
            const result = newEmptyItemResult();

            const canonicalRule = (canonicalRules || []).find((canonicalRule) =>
                analysis.url.match(canonicalRule.url)
            );

            if (analysis.isHtmlDocument && canonicalRule) {
                const match = analysis.url.match(canonicalRule.url);
                let expectedUrl = canonicalRule.expected;
                for (let i = 1; i < match.length; i++) {
                    expectedUrl = expectedUrl.replace(`(\$${i})`, match[i] || '');
                }

                if (analysis.canonicalUrl !== expectedUrl) {
                    result.passed = false;
                    result.messages.push(
                        newMessage(
                            analysis.url,
                            name,
                            `Expected canonicalUrl=${expectedUrl}, actual=${analysis.canonicalUrl}`
                        )
                    );
                }
            }

            return result;
        },
    };
};