Webpack-dev-server not bundling even after showing bundle valid message

I’ve got the issue resolved by myself. Silly as it sounds but the issue was with the publicPath under the output object. It should match the path property instead of just /build/, i.e.,

output: {
    path: path.join(__dirname, 'build/js'),
    publicPath: '/build/js', // instead of publicPath: '/build/' 
    filename: '[name].js'
},