After react(es6) packaged by webpack is executed in the browser, any variable declared globally will be reported as an error?

  node.js, question

I used react in the project, and the syntax was all written according to es6. Through webpack packaging and babel compilation, the packaged js file loaded in the browser was generated.
Js generated in this way does not have any problems during the execution of business logic, but when the browser console is opened and any variable is declared in it, an error will be reported, as follows:

Uncaught TypeError: Cannot assign to read only property 'toString' of function 'function bound(var_args)
 bracket
 return InjectedScriptHost.callFunction(func, thisObject, concat(args, sl...<omitted>... }'

Config is as follows:

module: {
 loaders: [
 bracket
 test: /\.react\.js$/,
 loader: 'babel',
 exclude: /node_modules/
 bracket
 bracket
 },
 plugins: [
 new webpack.DefinePlugin({
 "process.env": {
 NODE_ENV: JSON.stringify("production")
 bracket
 })
 bracket

Has anyone encountered a similar problem? I used the following version:
babel:6.5.2
react:0.14.5
webpack:1.12.14

The reason for this problem is that strict mode ('use strict';), modifying the prototype of JavaScript native type will report an error, and an error prompt will indicate that it has been modifiedfunction bound(var_args)ThetoString