koa-even-better-http-proxy/app/steps/decorateUserRes.js

http proxy middleware for Koa

import zlib from 'zlib';
import { asBuffer } from '../../lib/as';

const isResGzipped = (rspData, res) =>
    rspData?.byteLength > 0 && res.headers['content-encoding'] === 'gzip';

const zipOrUnzip = (method) => (rspData, res) =>
    isResGzipped(rspData, res) ? zlib[method](rspData) : rspData;

const maybeUnzipResponse = zipOrUnzip('gunzipSync');
const maybeZipResponse = zipOrUnzip('gzipSync');

const verifyBuffer = (rspd, reject) => {
    if (Buffer.isBuffer(rspd)) return null;
    return reject(
        new Error('userResDecorator should return string or buffer as data')
    );
};

const updateHeaders = (ctx, rspdBefore, rspdAfter, reject) => {
    if (!ctx.headerSent) {
        ctx.set('content-length', rspdAfter.length);
    } else if (rspdAfter.length !== rspdBefore.length) {
        const error =
            '"Content-Length" is already sent,' +
            'the length of response data can not be changed';
        reject(new Error(error));
    }
};

export default (container) => {
    if (container.user.ctx.status === 504) {
        return Promise.resolve(container);
    }

    const resolverFn = container.options.userResDecorator;
    if (!resolverFn) {
        return Promise.resolve(container);
    }

    const proxyResData = maybeUnzipResponse(
        container.proxy.resData,
        container.proxy.res
    );
    const proxyRes = container.proxy.res;
    const { ctx } = container.user;

    return Promise.resolve(resolverFn(proxyRes, proxyResData, ctx)).then(
        (modifiedResData) =>
            new Promise((resolve, reject) => {
                const rspd = asBuffer(modifiedResData, container.options);
                verifyBuffer(rspd, reject);
                updateHeaders(ctx, proxyResData, rspd, reject);
                // eslint-disable-next-line no-param-reassign
                container.proxy.resData = maybeZipResponse(
                    rspd,
                    container.proxy.res
                );
                resolve(container);
            })
    );
};