How to make a bocking Request in NodeJS inside a http-proxy-middleware function?

Question

I have a proxy server which modifies a specific type of POST Request before forwarding it to the Server.

The Modification requires me to make a request to another server in order to get the new value that will be put in place of the old one in the POST Request.

I can't get http-proxy-middleware 's onProxyReq function to block until I get the new value.

Anything will help. Thanks.

Code:

const options = {
    target: 'http://localhost:8998',
    changeOrigin: true,
    onProxyReq: (proxyReq, req, res) => {
        if (req.path === '/batches' && req.body && req.body.file) {
            request.post('MAKES REQUEST TO ANOTHER SERVER FOR NEW VALUE',{json: {}},(error,res,body)=>{
                req.body.url=body.url;
            });
            const bodyData = JSON.stringify(req.body);
            proxyReq.setHeader('Content-Type', 'application/json');
            proxyReq.setHeader('Content-Length', Buffer.byteLength(bodyData));
            proxyReq.write(bodyData);
        }else{
            const bodyData = JSON.stringify(req.body);
            proxyReq.setHeader('Content-Type', 'application/json');
            proxyReq.setHeader('Content-Length', Buffer.byteLength(bodyData));
            proxyReq.write(bodyData);
        }
    },
};

app.use(createProxyMiddleware(options));

The problem is that the POST request is forwarded before the new value can be received from the request.post operation.

Answer 1

I think this can be solved by using another middleware before the proxy-middleware. In this additional middleware-handler you can check whether to make the external request or not and assign the new url to the req.body . Then, in the onProxyReq you just check if the req.body was altered and recalculate the content-length in that case. Something like (note I'm using superagent as request has been deprecated):

const superagent = require('superagent');

app.use('/batches', async (req, res, next) => {
    if(req.body && req.body.file) {
       const resp = await superagent('http://your-other-host');
       req.body.url = resp.body.url;
    }
    next();
});

app.use(createProxyMiddleware(options));

Then in your proxy-config you can do:

const options = {
    target: 'http://localhost:8998',
    changeOrigin: true,
    onProxyReq: (proxyReq, req, res) => {
        if (req.path === '/batches' && req.body && req.body.file) {
            const bodyData = JSON.stringify(req.body);
            proxyReq.setHeader('Content-Type', 'application/json');
            proxyReq.setHeader('Content-Length', Buffer.byteLength(bodyData));
            proxyReq.write(bodyData);
        }
    },
};