What code coverage tool to use for the javascript code in a Kotlin Multiplatform project?

For now, there are no integrated code coverage tools. But you can implement it manually using karma.config.d and https://karma-runner.github.io/0.8/config/coverage.html.

Note: you can do it only with browser target

How to setup: Necessary to add dependencies, ideally dev dependencies into test source set, but dev dependencies are possible only since 1.4-M3, so they can be replaced with usual npm

implementation(devNpm("istanbul-instrumenter-loader", "3.0.1"))
implementation(devNpm("karma-coverage-istanbul-reporter", "3.0.3"))

After that create js file in karma.config.d folder in the project folder

;(function(config) { // just IIFE to protect local variabled
    const path = require("path") // native Node.JS module
    config.reporters.push("coverage-istanbul")
    config.plugins.push("karma-coverage-istanbul-reporter")
    config.webpack.module.rules.push(
        {
            test: /\.js$/,
            use: {loader: 'istanbul-instrumenter-loader'},
            include: [path.resolve(__dirname, '../module-name/kotlin/')] // here is necessary to use module-name in `build/js/packages`
        }
    )
    config.coverageIstanbulReporter = {
        reports: ["html"]
    }
}(config));

It works with Kotlin code (but honestly report is arguable) but anyway, it provides both statistics for js and Kt files, but for js indicate 0%.

I created a feature request: https://youtrack.jetbrains.com/issue/KT-40460

Update: The HTML file with results is in build/js/packages/{$module-name}-test/coverage/index.html. You can run build or browserTest task.

NOTE: If you on Windows you have to change include: [path.resolve(__dirname, '../module-name/kotlin/')] to include: [path.resolve(__dirname, '..\\module-name\\kotlin\\')]