Mastering JavaScript Testing with Chai

Test Expectations with Chai

Chai is a popular testing library that helps you make assertions about code behavior. It comes in many syntactic flavors, allowing you to choose the one that suits your needs. For example, if you want to assert that a variable is a string, you can use the expect() function.

const str = 'hello';
expect(str).to.be.a('string');

Handling Unexpected Inputs

As developers, we’re good at ensuring our code works when inputs are correct. However, the real challenge lies in handling unexpected or failing inputs. Chai’s throw() helper allows you to assert that code should throw an exception, making it easier to test for edge cases.

function divide(a, b) {
  if (b === 0) {
    throw new Error('Cannot divide by zero');
  }
  return a / b;
}

expect(() => divide(10, 0)).to.throw(Error, 'Cannot divide by zero');

Equality Checks with equal() and eql()

Chai provides two built-in functions for determining equality: equal() and eql(). While equal() checks for referential equality, eql() performs a deep equality check. Using these functions provides more informative test output compared to using simple conditional statements.

const obj1 = { a: 1, b: 2 };
const obj2 = { a: 1, b: 2 };

expect(obj1).to.equal(obj1); // true
expect(obj1).to.equal(obj2); // false

expect(obj1).to.eql(obj2); // true

Extending Chai with Plugins

Chai can be extended with plugins that provide additional functionality. One such plugin is dirty-chai, which allows you to write more robust assertions by throwing exceptions when syntax mistakes are made. Another useful plugin is sinon-chai, which enables you to make Sinon assertions with the Chai syntax.

Using eslint-plugin-chai-expect for Linting

If you’re using ESLint to lint your code, eslint-plugin-chai-expect can help prevent common syntax errors when working with Chai. This plugin ensures that your code is error-free and follows best practices.

• Install the plugin using npm: npm install eslint-plugin-chai-expect
• Add the plugin to your ESLint configuration: "plugins": {"chai-expect": "error"}

By mastering Chai, you can write more effective tests and ensure your code is robust and reliable.