Heroku build failing after tweaking postinstall script

Question

Well I am working on pull request on Heroku project I am experiencing following issue with it:

-----> Deleting 1 files matching .slugignore patterns. -----> Node.js app detected -----> Creating runtime environment

   NPM_CONFIG_LOGLEVEL=error
   NPM_CONFIG_PRODUCTION=false
   NODE_VERBOSE=false
   NODE_ENV=production
   NODE_MODULES_CACHE=true

-----> Installing binaries engines.node (package.json): 9.3.0 engines.npm (package.json): 5.5.1

   Resolving node version 9.3.0...
   Downloading and installing node 9.3.0...
   npm 5.5.1 already installed with node

-----> Restoring cache Loading 2 from cacheDirectories (default): - node_modules - bower_components (not cached - skipping) -----> Building dependencies Installing node modules (package.json) uglifyjs-webpack-plugin@0.4.6 postinstall /tmp/build_0205c32fc162b04a5bd36ce6d3e2a31a/GithubUsername-projName-45f1019/node_modules/webpack/node_modules/uglifyjs-webpack-plugin node lib/post_install.js projName@0.3.0 postinstall /tmp/build_0205c32fc162b04a5bd36ce6d3e2a31a/GithubUserName-projName-45f1019 webpack -p --config ./webpack.config.js --progress

  90% chunk assets pro       Version: webpack 3.10.0
   Time: 25301ms
   Asset       Size  Chunks                    Chunk Names
  ///All chunk files emitted details
   projname.js    1.27 MB       0  [emitted]  [big]  main
   favicon.ico      17 kB          [emitted]
   index.html    37.4 kB          [emitted]
   [14] ./src/script.js 3.92 kB {0} [built]
   //built details
   + 208 hidden modules

   ERROR in ./src/assetLoader.js
   Module not found: Error: Can't resolve './manifest' in '/tmp/build_0205c32fc162b04a5bd36ce6d3e2a31a/GithubUsername-projName-45f1019/src'
   @ ./src/assetLoader.js 1:0-34
   @ ./src/game.js
   @ ./src/script.js

   ERROR in proj.js from UglifyJs
   Unexpected token: name (Creature) [GithubUsername-projName.js:1,99245]
   Child html-webpack-plugin for "index.html":
   Asset     Size  Chunks                    Chunk Names

   [2] ./node_modules/html-webpack-plugin/lib/loader.js!./src/index.html 

[built] [4] (webpack)/buildin/global.js 509 bytes {0} [built] [5] (webpack)/buildin/module.js 517 bytes {0} [built] [6] ./assets/interface/AB.gif 82 bytes {0} [built] [7] ./assets/interface/ProjPage.png 82 bytes {0} [built] npm ERR! code ELIFECYCLE npm ERR! errno 2 npm ERR! projName@0.3.0 postinstall: webpack -p --config ./webpack.config.js --progress npm ERR! Exit status 2 npm ERR! npm ERR! Failed at the projName@0.3.0 postinstall script. npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

   npm ERR! A complete log of this run can be found in:
   npm ERR!     /app/.npm/_logs/2018-01-03T13_35_09_304Z-debug.log

-----> Build failed

   We're sorry this build is failing! You can troubleshoot common issues here:
   https://devcenter.heroku.com/articles/troubleshooting-node-deploys

   If you're stuck, please submit a ticket so we can help:
   https://help.heroku.com/

   Love,
   Heroku
     !     Push rejected, failed to compile Node.js app.  !     Push failed

here is code for:

snippet of package.json

{
    "dependencies": {
        "compression": "^1.6.2",
        "express": "^4.13.4",
        "html-webpack-plugin": "^2.30.1",
        "jquery": "1.12.4",
        "jquery-mousewheel": "3.1.13",
        "jquery-ui": "^1.12.1",
        "jquery.kinetic": "2.2.4",
        "jquery.transit": "0.9.12",
        "phaser-ce": "2.7.10",
        "socket.io": "^1.3.5",
        "socket.io-client": "1.3.5"
    },
    "devDependencies": {
        "css-loader": "^0.28.7",
        "eslint": "^4.12.1",
        "expose-loader": "^0.7.4",
        "extract-text-webpack-plugin": "^3.0.2",
        "file-loader": "^1.1.5",
        "less": "^2.7.3",
        "less-loader": "^4.0.5",
        "prettier": "^1.8.2",
        "style-loader": "^0.19.0",
        "uglifyjs-webpack-plugin": "^1.1.2",
        "webpack": "^3.8.1",
        "webpack-merge": "^4.1.1"
    },

    "scripts": {
        "start": "node server.js",
        "postinstall": "webpack -p --config ./webpack.config.js --progress",
        "build": "yarn run generateManifest && webpack --env.production",
        "build:dev": "yarn run generateManifest && webpack",
        "dev": "webpack --watch",
        "lint": "eslint src/**/*.js",
        "generateManifest": "node manifestGenerator.js"
    }
}

Server.js

// Setup basic express server
var compression = require('compression');
var express = require('express');
var app = express();
var server = require('http').createServer(app);
var io = require('socket.io')(server);
var port = process.env.PORT || 8080;
var ip = process.env.IP || '127.0.0.1';
var gameManager = require('./server/gamemanager.js');
var qManager = require('./server/queuemanager.js');

// Setup the game queue and connection details
io.on('connection', function(session) {
    console.log('a user connected');

    // Store the username in the socket session for this client
    var username = makeid();
    session.username = username;

    // Add user to the queue
    qManager.addToQueue(session);

    session.on('disconnect', function() {
        console.log('user disconnected');
        qManager.removeFromQueue(session);
    });

    // Send user the username
    session.emit('login', session.username);

});


// Listen for server, and use static routing for deploy directory
server.listen(port, function() {
    console.log('Server listening at port %d', port);
});

app.use(express.static('./deploy', {
    maxAge: 86400000
}));

webpack.config.js code

 const path = require('path');
const webpack = require('webpack');
const merge = require('webpack-merge');
const HtmlWebpackPlugin = require('html-webpack-plugin');
const UglifyJSPlugin = require('uglifyjs-webpack-plugin');

// Are we in production
const production = process.env.production;

const baseSettings = {
    entry: path.resolve(__dirname, 'src', 'script.js'),
    output: {
        path: path.resolve(__dirname, 'deploy/'),
        filename: 'projname.js'
    },
    module: {
        rules: [
            {
                test: /\.less$/,
                use: [
                    'style-loader',
                    'css-loader',
                    'less-loader',
                ]
            },
            {
                test: /\.css$/,
                use: [
                    'style-loader',
                    'css-loader',
                ]
            },
            {
                test: /\.(png|jpg|gif|svg|ogg|ico|cur|woff|woff2)$/,
                use: [
                    'file-loader'
                ]
            }
        ]
    },
    resolve: {
        alias: {
            assets: path.resolve(__dirname, 'assets/'),
            modules: path.join(__dirname, "node_modules")
        }
    },
    plugins: [
        new HtmlWebpackPlugin({
            template: path.resolve(__dirname, 'src', 'index.html'),
            favicon: path.resolve(__dirname, 'assets', 'favicon.ico')
        }),
    ]
}

const prodSettings = {
    plugins: [
        new UglifyJSPlugin(),
        new webpack.DefinePlugin({
            'process.env.NODE_ENV': JSON.stringify('production')
        })
    ]
}

const devSettings = {
    devtool: 'cheap-module-eval-source-map'
}

// Create either a production or development build depending on the `production` env setting
module.exports = merge(baseSettings, production ? devSettings : prodSettings);

I am not able to understand what the issue with web pack is their something wrong with my postinstall script? Am i missing any rules while tweaking script?

I tried applying some solutions from SO and Heroku documentation but nothing working ...Please Help !!! Thank you in advance!!! Feel free to comment for any other info.!!!