Chai Plugin for Snapshot Testing with Karma

Snapshot Serialization

This plugin is using the same serialization module that is used in Jest library.

Snapshot Format

Snapshot can be stored in different formats. Right now there are two formats supported: md and indented-md.

Markdown Format

This format is preferred when you specify language for code blocks in an assertion plugin. With this format, code editors will automatically highlight syntax of code blocks.

# `src/html.js`

## `Sub Suite`

####   `HTML Snapshot`

  <span />

Indented Markdown Format

# `src/html.js`

## `Sub Suite`

####   `HTML Snapshot`

      <span />

Snapshot File Path

Snapshot file path is extracted from the name of the root suit cases and stored alongside with a tested files in a __snapshots__ directory.

Snapshot file path can be changed by providing a custom pathResolver in snapshot config.

Usage Example

$ npm install karma karma-webpack karma-sourcemap-loader karma-snapshot karma-mocha \
              karma-mocha-snapshot karma-mocha-reporter karma-chrome-launcher mocha \
              chai chai-karma-snapshot webpack --save-dev

Karma configuration:

// karma.conf.js
const webpack = require("webpack");

module.exports = function (config) {
    browsers: ["ChromeHeadless"],
    frameworks: ["mocha", "snapshot", "mocha-snapshot"],
    reporters: ["mocha"],
    preprocessors: {
      "**/__snapshots__/**/*.md": ["snapshot"],
      "__tests__/index.js": ["webpack", "sourcemap"]
    files: [

    colors: true,
    autoWatch: true,

    webpack: {
      devtool: "inline-source-map",
      performance: {
        hints: false

    webpackMiddleware: {
      stats: "errors-only",
      noInfo: true

    snapshot: {
      update: !!process.env.UPDATE,
      prune: !!process.env.PRUNE,

    mochaReporter: {
      showDiff: true,

    client: {
      mocha: {
        reporter: "html",
        ui: "bdd",

Source file:

// src/index.js

export function test() {
  return "Snapshot Test";

Test file:

// __tests__/index.js
import { use, expect, assert } from "chai";
import { matchSnapshot } from "chai-karma-snapshot";
import { test } from "../src/index.js";

describe("src/index.js", () => {
  it("check snapshot", () => {
    // 'expect' syntax:
    // 'assert' syntax:

Run tests:

$ karma start

Update snapshots:

$ UPDATE=1 karma start --single-run

Prune snapshots:

$ PRUNE=1 karma start --single-run


function resolve(basePath, suiteName) {
  return path.join(basePath, "__snapshots__", suiteName);

  snapshot: {
    update: true,           // Run snapshot tests in UPDATE mode (default: false)
    prune: false,            // Prune snapshots for removed tests (default: true)
    format: "indented-md",  // Snapshot format (default: md)
    checkSourceFile: true,  // Checks existince of the source file associated with tests (default: false)
    pathResolver: resolve,  // Custom path resolver

Custom Snapshot Format

Snapshot config option format also works with custom serialization formats. Custom snapshot serializer should have interface:

interface SnapshotSerializer {
  serialize: (name: string, suite: SnapshotSuite) => string,
  deserialize: (content: string) => { name: string, suite: SnapshotSuite },