how to do middleware proxy only client code example

Example: http-proxy-middleware option.onProxyReq:

'use strict';

var express = require('express');
var router = express.Router();

var proxy_filter = function (path, req) {
    return path.match('^/docs') && ( req.method === 'GET' || req.method === 'POST' );
};

var proxy_options = {
    target: 'http://localhost:8080',
    pathRewrite: {
        '^/docs' : '/java/rep/server1' // Host path & target path conversion
    },
    onError(err, req, res) {
        res.writeHead(500, {
            'Content-Type': 'text/plain'
        });
        res.end('Something went wrong. And we are reporting a custom error message.' + err);
    },
    onProxyReq(proxyReq, req, res) {
        if ( req.method == "POST" && req.body ) {
            // Add req.body logic here if needed....

           // ....

            // Remove body-parser body object from the request
            if ( req.body ) delete req.body;

            // Make any needed POST parameter changes
            let body = new Object();

            body.filename = 'reports/statistics/summary_2016.pdf';
            body.routeid = 's003b012d002';
            body.authid = 'bac02c1d-258a-4177-9da6-862580154960';

            // URI encode JSON object
            body = Object.keys( body ).map(function( key ) {
                return encodeURIComponent( key ) + '=' + encodeURIComponent( body[ key ])
            }).join('&');

            // Update header
            proxyReq.setHeader( 'content-type', 'application/x-www-form-urlencoded' );
            proxyReq.setHeader( 'content-length', body.length );

            // Write out body changes to the proxyReq stream
            proxyReq.write( body );
            proxyReq.end();
        }
    }
};

// Proxy configuration
var proxy = require( 'http-proxy-middleware' )( proxy_filter, proxy_options );

/* GET home page. */
router.get('/', function(req, res, next) {
    res.render('index', { title: 'Node.js Express Proxy Test' });
});

router.all('/document', proxy );

module.exports = router;