When I run ‘wepack -p’, there’re many warnings. WARNING in bundle.js from UglifyJs

My webpack.config.js:

var webpack = require('webpack');
var path = require('path');

module.exports = {
    devtool: 'inline-source-map', 
    entry: [
        'webpack-dev-server/client?http://127.0.0.1:8080/',
        'webpack/hot/only-dev-server',
        './app'
    ],
    output: {
        path: path.join(__dirname, 'build'),
        filename:'bundle.js'
    },
    resolve: {
        modulesDirectories: ['node_modules', 'app'],
        extensions: ['','.js']
    },
    module:{
        loaders: [
            {
                test:/\.jsx?$/,
                exclude:/node_modules/,
                loaders:[
                    'react-hot','babel?presets[]=react,presets[]=es2015'
                ]
            }
        ]
    },
    plugins: [
        new webpack.HotModuleReplacementPlugin(),
        new webpack.NoErrorsPlugin(),
        new webpack.optimize.UglifyJsPlugin()
    ]

};

And when I run ‘wepack -p’, there’re many warnings:

WARNING in bundle.js from UglifyJs
Condition always true [.//react/lib/ReactMount.js:764,0]
Dropping side-effect-free statement [.//react-router/lib/deprecateObjectPropert
ies.js:65,0]
Side effects in initialization of unused variable _ret [.//react-router/lib/dep
recateObjectProperties.js:63,0]
Condition always true [.//react/lib/findDOMNode.js:46,0]
Condition always true [.//react/lib/instantiateReactComponent.js:80,0]
Dropping unreachable code [.//react/lib/shouldUpdateReactComponent.js:40,0]
Condition always true [.//react/lib/traverseAllChildren.js:158,0]
Condition always true [.//react-router/lib/Redirect.js:97,0]
Side effects in initialization of unused variable _reactRouter [./modules/App.js
:2,0]
Condition always true [.//react-router/lib/IndexRedirect.js:58,0]
Condition always true [.//react-router/lib/IndexRoute.js:55,0]
Condition always true [.//react-router/lib/Route.js:50,0]
Dropping side-effect-free statement [.//react-router/lib/getComponents.js:49,0]

Side effects in initialization of unused variable _ret2 [.//react-router/lib/ge
tComponents.js:47,0]
Condition always true [.//react-router//history/lib/createMemoryHistory.js:67,
0]
Condition always true [.//react/lib/ReactDOMComponent.js:907,0]
Dropping side-effect-free statement [.//react/lib/ReactEventListener.js:72,0]
Dropping unused function handleTopLevelWithPath [.//react/lib/ReactEventListene
r.js:98,0]
Dropping unused variable DOCUMENT_FRAGMENT_NODE_TYPE [./~/react/lib/ReactEventLi
stener.js:26,0]