Chai Assertion Library

Common errors

• Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'chai/lib/expect' imported from ...

  cause Attempting to import internal `lib/*.js` files directly after Chai v6.0.0, which bundled the library into a single `index.js`.
  fix Change the import path to `import { expect } from 'chai';` or `import { assert } from 'chai';`.

• TypeError: (0 , chai__WEBPACK_IMPORTED_MODULE_0__.should) is not a function

  cause Using Chai's `should` assertion style without calling `should()` after importing it, which is necessary to extend `Object.prototype`.
  fix Add `should();` immediately after the import statement: `import { should } from 'chai'; should();`.

• ReferenceError: require is not defined

  cause Attempting to use the CommonJS `require` function in an ECMAScript Module (ESM) file context (e.g., a `.mjs` file or a project with `"type": "module"` in `package.json`).
  fix Update your code to use ESM `import` statements, for example, `import { expect } from 'chai';` instead of `const expect = require('chai').expect;`.

Warnings

• breaking Chai v6.0.0 introduced a breaking change where direct imports from internal `lib/*.js` files are no longer supported. The library is now bundled into a single `index.js`.
  Fix: Update your imports to reference the main `chai` package directly, e.g., `import { expect } from 'chai';` instead of `import { expect } from 'chai/lib/expect';`.
• gotcha When using Chai's `should` assertion style, you must explicitly call `should()` after importing it to extend `Object.prototype` and enable the assertion syntax.
  Fix: After `import { should } from 'chai';`, add `should();` on the next line.
• gotcha Importing a Chai assertion style globally (e.g., `import 'chai/register-expect';`) modifies the global scope. While convenient for quick setups, this can lead to unexpected side effects or conflicts in larger projects.
  Fix: Prefer importing assertion styles locally (e.g., `import { expect } from 'chai';`) to avoid global pollution and ensure better test isolation.

Install

• npm install chai npm
• yarn add chai yarn
• pnpm add chai pnpm

Imports

• expect
  wrong:

  const expect = require('chai').expect;

  correct:

  import { expect } from 'chai';

• assert
  wrong:

  const assert = require('chai').assert;

  correct:

  import { assert } from 'chai';

• should
  wrong:

  import { should } from 'chai';

  correct:

  import { should } from 'chai';
  should();

Quickstart

This quickstart demonstrates basic assertions using Chai's Expect style, including numerical comparisons, boolean checks, and deep object equality.

import { expect } from 'chai';

const add = (a, b) => a + b;
const isPositive = (num) => num > 0;

// Basic assertions
expect(add(2, 3)).to.equal(5);
expect(add(-1, 5)).to.be.above(0);
expect(isPositive(10)).to.be.true;

// Deep equality for objects
expect({ a: 1, b: 2 }).to.deep.equal({ a: 1, b: 2 });