window not defined error when using extract-text-webpack-plugin React

You may want to use style-loader as a before argument in extract function.

Here’s the native implementation:

    ExtractTextPlugin.extract = function(before, loader, options) {
        if(typeof loader === "string") {
            return [
                ExtractTextPlugin.loader(mergeOptions({omit: before.split("!").length, extract: true, remove: true}, options)),
                before,
                loader
            ].join("!");
        } else {
            options = loader;
            loader = before;
            return [
                ExtractTextPlugin.loader(mergeOptions({remove: true}, options)),
                loader
            ].join("!");
        }
    };

So basicaly what you need to do is:

{
    test: /\.sass$/,
    exclude: /node_modules/,
    loader: ExtractTextPlugin.extract('style-loader', 'css!sass?indentedSyntax=true&sourceMap=true')
},

if you for example use sass.