Webpack ZoneAwareError - Expected 'styles' To Be An Array Of Strings (no Preprocessor)

- 1 answer

I am trying to configure Webpack correctly. I am using this blog post from Microsoft as basis.

Currently, I am getting an exception in the console, saying Expected 'styles' to be an array of strings. I researched the error more deeply and found that in most cases this is caused by a missing loader in the webpack.config.js file or bad configuration, but the project is working fine until I try to create an embedded app of it, in Shopify. Keep in mind that I am currently not using any preprocessors!

So my questions are:

  1. Can this error be caused by the fact that the script is runned in an iFrame?
  2. Am I doing something wrong in the configuration?

Here is my webpack.config.js file:

const path = require('path');
const webpack = require('webpack');
const merge = require('webpack-merge');
const CheckerPlugin = require('awesome-typescript-loader').CheckerPlugin;

module.exports = (env) => {
    // Configuration in common to both client-side and server-side bundles
    const isDevBuild = !(env &&;
    const sharedConfig = {
        stats: { modules: false },
        context: __dirname,
        resolve: { extensions: ['.ts', '.js'] },
        output: {
            filename: '[name].js',
            publicPath: '/dist/' // Webpack dev middleware, if enabled, handles requests for this URL prefix
        module: {
            rules: [
                { test: /\.ts$/, include: [/ClientApp/, /Plugins/], use: ['awesome-typescript-loader?silent=true', 'angular2-template-loader'] },
                { test: /\.html$/, use: 'html-loader?minimize=false' },
                { test: /\.css$/, use: ['to-string-loader', 'css-loader'] },
                { test: /\.(png|jpg|jpeg|gif|svg)$/, use: 'url-loader?limit=25000' }
        plugins: [new CheckerPlugin()]

    // Configuration for client-side bundle suitable for running in browsers
    const clientBundleOutputDir = './wwwroot/dist';
    const clientBundleConfig = merge(sharedConfig, {
        entry: { 'bundle': './ClientApp/boot-client.ts' },
        output: { path: path.join(__dirname, clientBundleOutputDir) },
        plugins: [
            new webpack.DllReferencePlugin({
                context: __dirname,
                manifest: require('./wwwroot/dist/vendor-manifest.json')
        ].concat(isDevBuild ? [
            // Plugins that apply in development builds only
            new webpack.SourceMapDevToolPlugin({
                filename: '[file].map', // Remove this line if you prefer inline source maps
                moduleFilenameTemplate: path.relative(clientBundleOutputDir, '[resourcePath]') // Point sourcemap entries to the original file locations on disk
        ] : [
                // Plugins that apply in production builds only
                new webpack.optimize.UglifyJsPlugin()

    // Configuration for server-side (prerendering) bundle suitable for running in Node
    const serverBundleConfig = merge(sharedConfig, {
        resolve: { mainFields: ['main'] },
        entry: { 'main-server': './ClientApp/boot-server.ts' },
        plugins: [
            new webpack.DllReferencePlugin({
                context: __dirname,
                manifest: require('./ClientApp/dist/vendor-manifest.json'),
                sourceType: 'commonjs2',
                name: './vendor'

    output: {
        libraryTarget: 'commonjs',
        path: path.join(__dirname, './ClientApp/dist')
    target: 'node',
    devtool: 'source-map'

return [clientBundleConfig, serverBundleConfig];


Here is the stack trace of the JS exception that I am getting:

Error: Expected 'styles' to be an array of strings.
    at assertArrayOfStrings (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:70123:15) [<root>]
    at CompileMetadataResolver.getNonNormalizedDirectiveMetadata (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:32189:101) [<root>]
    at CompileMetadataResolver._loadDirectiveMetadata (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:32121:23) [<root>]
    at https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:32322:54 [<root>]
    at Array.forEach (native) [<root>]
    at CompileMetadataResolver.loadNgModuleDirectiveAndPipeMetadata (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:32321:41) [<root>]
    at https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:72595:58 [<root>]
    at Array.forEach (native) [<root>]
    at JitCompiler._loadModules (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:72594:43) [<root>]
    at JitCompiler._compileModuleAndComponents (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:72549:52) [<root>]
    at JitCompiler.compileModuleAsync (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:72515:21) [<root>]
    at PlatformRef_._bootstrapModuleWithZone (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:53194:25) [<root>]
    at PlatformRef_.bootstrapModule (https://localhost/dist/vendor.js?v=rUYiw0agrE4eOpKoyLar1iLT1ql6s5KESRzWj-HWhyc:53169:21) [<root>]
    at HTMLDocument.bootApplication (https://localhost/dist/bundle.js?v=5y5PspNLD-tK9PXgSUUIscip2N-vrKWGkiMyAtnQdaI:4813:46) [<root>]

Does anyone has any idea of what could be the cause of this?



I found the problem. :)

It was copy-paste error. In two of my components the initialization was:

  selector: 'app', 
  template: require('./app.component.html'), 
  styles: require('./app.component.css') 

Like this. Notice that the styles property isn't an array. Apparently, it should be.