post-request-handler/handler.js

// === Handler ===
export const postHandler = async (req, res) => {
    try {
        const modified = processPostRequest(req.body?.response);
        return res.status(200).json(modified);
    } catch (err) {
        return handleError(err, res);
    }
};

// === Core Processing ===
function processPostRequest(response) {
    if (!response || typeof response !== 'object') {
        throw new InvalidJsonContentTypeError('Missing or invalid response body', 400);
    }

    const modified = postRequestHandlerWrapper(response);
    return modified;
}

// === Unified Error Handler ===
function handleError(err, res) {
    const isJsonTypeError = err instanceof InvalidJsonContentTypeError ||
        err?.message?.includes('Body content-type is not valid JSON');

    const status = isJsonTypeError ? err.statusCode || 415 : 500;

    return res.status(status).json({
        error: err.name || 'InternalServerError',
        message: err.message || 'Unexpected error occurred',
        ...(err.details && { details: err.details }),
    });
}
let currentResponse = null;
let postContext = null;

function createPostContext() {
    return {
        addedHeaders: {},
        removedHeaders: [],
        modifiedBody: null,
        modifiedStatus: null,
    };
}

export function postRequestHandlerWrapper(response) {
    postContext = createPostContext();
    currentResponse = response;
    postRequestHandler();
    const ctx = postContext;
    postContext = null;
    currentResponse = null;
    return ctx;
}

export function postRequestHandler() {
    setHeader('x-post-transform', 'applied');
    removeHeader('x-unwanted-header');

    const body = getJsonBody();
    if (body) {
        body.modifiedAt = new Date().toISOString();
        body.transformed = "transformed***********";
        setJsonBody(body);
    }

    setStatus(202);
}
export function setHeader(key, value) {
    if (!currentResponse?.headers) currentResponse.headers = {};
    currentResponse.headers[key.toLowerCase()] = value;
    postContext?.addedHeaders && (postContext.addedHeaders[key.toLowerCase()] = value);
}

export function removeHeader(key) {
    if (currentResponse?.headers?.[key.toLowerCase()] !== undefined) {
        delete currentResponse.headers[key.toLowerCase()];
        postContext?.removedHeaders?.push(key.toLowerCase());
    }
}

export function getHeader(key) {
    if (!currentResponse?.headers) return null;
    return currentResponse.headers[key.toLowerCase()] || null;
}

export function getJsonBody() {
    if (!currentResponse?.body) return null;

    const contentType = currentResponse.contentType || currentResponse.headers?.['content-type'] || '';

    if (!contentType.includes('application/json')) {
        throw new InvalidJsonContentTypeError();
    }

    try {
        return typeof currentResponse.body === 'string'
            ? JSON.parse(currentResponse.body)
            : currentResponse.body;
    } catch (err) {
        throw new InvalidJsonContentTypeError('Failed to parse JSON body', 415, { originalError: err });
    }
}

export function setJsonBody(json) {
    const contentType = currentResponse.contentType || currentResponse.headers?.['content-type'] || '';

    if (!contentType.includes('application/json')) {
        throw new InvalidJsonContentTypeError();
    }

    try {
        currentResponse.body = JSON.stringify(json);
        postContext && (postContext.modifiedBody = json);
    } catch (err) {
        throw new InvalidJsonContentTypeError('Failed to stringify JSON body', 415, { originalError: err });
    }
}

export function setStatus(statusCode) {
    if (typeof statusCode === 'number') {
        currentResponse.status = statusCode;
        postContext && (postContext.modifiedStatus = statusCode);
    }
}
export class InvalidJsonContentTypeError extends Error {
    constructor(message = 'Body content-type is not valid JSON', statusCode = 415, details = {}) {
        super(message);
        this.name = 'InvalidJsonContentTypeError';
        this.statusCode = statusCode;
        this.details = details;
    }
}