Ignoring coverage on typescript decorators

Question

I'm using Typescript to create some web components with Lit element. Webpack to bundle and unit tests with karma.

I've noticed that the coverage report is all out:

• showing uncovered branches and statements on decorators
• coverage for lines following decorators are reported incorrectly

I've tried adding /* istanbul ignore next */ before decorators, however tsc removes this.

Any ideas on how to update configuration to prevent decorators from being included in coverage reports?

Source:

import {customElement, html, LitElement, property} from "lit-element";
import Style from "./style.less";

@customElement("webcom-voorbeeld")
export class WebcomVoorbeeld extends LitElement {

    @property({type: String, attribute: "attribute"})
    attribute = "default";

    @property({type: Number})
    property = 0;

    static get styles() {
        return [Style];
    }

    constructor() {
        super();
    }

    render() {
        return html
            `<div class="webcom webcom-voorbeeld">
                <p class="heading font-bold webcom-clr-blue-02">New Voor-Beeld component</p>
                <div class="slot copy"><slot></slot></div>
                <button class="webcom-cta-btn" @click="${this.handleClick}">${this.attribute}</button>
            </div>`;
    }

    private handleClick() {
        this.property++;

        const event = new CustomEvent("webcom-event-name", {
            detail: {
                message: {
                    clicked: this.property,
                },
            },
            bubbles: true,
        });
        this.dispatchEvent(event);
    }
}

Webpack config

const {CleanWebpackPlugin} = require('clean-webpack-plugin');

module.exports = {
    context: __dirname,
    entry: './src/webcom-voorbeeld.ts',
    output: {
        filename: 'webcom-voorbeeld.js',
    },
    devtool: 'inline-source-map',
    mode: 'development',
    watchOptions: {
        ignored: [
            'node_modules/**',
            'test/**'
        ]
    },
    module: {
        rules: [
            {
                test: /\.ts$/,
                exclude: /node_modules/,
                use: 'ts-loader'
            },
            {
                test: /\.ts$/,
                exclude: /(node_modules|\.spec\.ts$|test-utils.ts$)/,
                loader: 'istanbul-instrumenter-loader',
                enforce: 'post',
                options: {
                    esModules: true
                }
            },
            {
                test: /\.woff2$/,
                loader: 'url-loader'
            },
            {
                test: /\.(woff|eot|ttf)$/,
                loader: 'file-loader'
            },
            {
                test: /\.(svg|png|jpg|gif)$/i,
                use: [
                    {
                        loader: 'url-loader',
                        options: {
                            limit: 8192,
                        },
                    },
                ],
            },
            {
                test: /\.less|\.css$/,
                use: [
                    {
                        loader: 'lit-scss-loader',
                        options: {
                            minify: false, 
                        },
                    },
                    'extract-loader',
                    {
                        loader: 'css-loader',
                    },
                    {
                        loader: 'less-loader',
                        options: {
                            lessOptions: {
                                strictMath: true,
                                noIeCompat: true,
                            }
                        }
                    },
                    'postcss-loader'
                ],
            },
        ],
    },
    resolve: {
        extensions: ['.tsx', '.ts', '.js'],
    },
    optimization: {
        minimize: false
    },
    plugins: [
        new CleanWebpackPlugin()
    ],
    stats: {
        colors: true,
        hash: false,
        version: true,
        timings: true,
        assets: false,
        chunks: false,
        modules: false,
        reasons: true,
        children: false,
        source: false,
        errors: true,
        errorDetails: true,
        warnings: true,
        publicPath: false,
    },
    bail: true
};

Coverage report:

Crannog