AWS CloudFront function for SPA, SSG and SEO. Redirects www URL to non-www URI, also adds missing `/index.html` to the link

redirect_www_add_index_html.js

/**
 * Based on this thread https://github.com/aws-samples/amazon-cloudfront-functions/issues/11
 */
function getQueryParamsAsString(querystring) {
    if (!querystring || querystring.length < 1) {
        return ''; // There is no query params
    }
    
    const str = [];
    for (const param in querystring) {
        const query = querystring[param];
        const multiValue = query.multiValue;
        if (multiValue) {
            str.push(multiValue.map((item) => param + '=' + item.value).join('&'));
        } else if (query.value === '') {
            str.push(param);
        } else {
            str.push(param + '=' + query.value);
        }
    }
    if (str.length < 1) {
        return ''; // No query parms were parsed
    }
    
    const result = '?' + str.join('&');
    return result
}

/**
 * CloudFront function to redirect `https://www.*` and `http://www.*` URI to `to https://*`
 * Also adds missing `/index.html` to the end of URI, useful for SPA and SSG websites.
 * Supports query params for redirects. 
 * @example: `http://www.xxx.com/page/?param=1` -> `https://xxx.com/page/?param=1`
 * @example: `https://www.xxx.com/page/?param=1` -> `https://xxx.com/page/?param=1`
 * @example: `https://xxx.com/page/` -> `https://xxx.com/page/index.html`
 * @example: `https://xxx.com/page` -> `https://xxx.com/page/index.html`
 * @example: `https://xxx.com/page/?ref=user1` -> `https://xxx.com/page/index.html?ref=user1`
 * @example: `https://xxx.com/page?ref=user1` -> `https://xxx.com/page/index.html?ref=user1`
 */
function handler(event) {
    const request = event.request;
    const uri = request.uri;

    // Redirect .www to https + non-www
    if (request.headers["host"] && request.headers["host"].value.startsWith("www.")) {
        const queryParamsToApplied = getQueryParamsAsString(request.querystring)
        const response = {
            statusCode: 301,
            statusDescription: 'Moved Permanently',
            headers: {
                'location': { value: 'https://' + request.headers["host"].value.replace('www.', '') + uri + queryParamsToApplied  }
            }
        };
        return response; // Stop processing request here
    }
    
    // Add missing /index.html part
    if (uri.endsWith('/')) {
        // Check whether the URI is missing a file name.
        request.uri += 'index.html';
    } else if (!uri.includes('.')) {
        // Solution for the root and folders
        request.uri += '/index.html';
    }

    return request;
}