Welcome to Chai

We are glad that you have decided to give Chai.js a try! If this is your first visit, get acquainted with the basics, such as installation and our included assertion styles.

The Basics

Making Plugins

After you have a solid knowledge of what is included in Chai, the next step is learning how to extend Chai through plugins. Chai is infinitely more powerful than what is included, limited only by what you want to achieve. The Plugin API is also intended as a way to simplify testing by providing users a way to encapsulate common assertions for repeat use.

Exposing Globals in Plugins

When creating a Chai plugin, it’s possible to expose globals that can be used across multiple files. Here’s how to do it sustainably:

Good Practice

Prefer exporting any global in the module record so it can be imported directly instead of adding it as a property in the chai object:

// An example of a good plugin:

export const myGlobal = {...};

export default function myPlugin(chai, utils) {

Potential Issues

Avoid exposing globals only through chai.use() without making them available for import, as this can lead to issues when trying to use the global across multiple files:

// An example of a plugin which may have issues:

const myGlobal = {...};

export default function myPlugin(chai, utils) {
  chai.myGlobal = myGlobal;
// Another example of a plugin which may have issues:

export default function myPlugin(chai, utils) {
  chai.myGlobal = {...};

Guard against multiple calls to use(..)

In certain situations the use(..) function could be called multiple times with your plugin. For a lot of plugins this won’t be a issue but it’s considered best practise to check if the plugin has been applied already.

Here’s a contrived example of how you might implement a check in your plugin but the actual implementation is left up to the plugin author.

import * as chai from 'chai';

let overwritten = false;

function somePlugin(base) {
  if (!overwritten) {
    base.util.overwriteMethod(base.Assertion.prototype, "equal", function (_super) {
      return function(...args) {
        console.log("Called!"); // log something out

        return _super.call(this, ...args);
    overwritten = true;


chai.expect(123).to.equal(123); // Logs `Called!` only once

By following these guidelines, you can create Chai plugins that are easy to use and maintain.