Assertions in Mocha
One of the most common criticisms of Mocha is that it doesn't include any assertion tools.
It is impossible to write meaningful tests without some sort of assertions.
The idea is that Node's built-in assert module is good enough for most use cases.
const assert = require('assert');
describe('add', function() {
it('adds two numbers', function() {
assert.equal(add(1, 2), 3);
});
it('concatenates strings', function() {
assert.equal(add('hello ', 'world'), 'hello world');
});
});
function add(a, b) {
return a + b;
}We recommend using Node's built-in assert module with Mocha tests by default.
assert.ok(), assert.equal(), assert.strictEqual(), assert.throws(), assert.rejects(), and assert.deepStrictEqual() cover every assertion we've wanted to write over the last several years.
Alternative Approach: Chai
Chai is an assertion library that is commonly used with Mocha.
Chai exports several different flavors of assertions.
The expect flavor is the most common.
const { expect } = require('chai');
describe('sum()', function() {
it('adds two numbers', function() {
// `expect()` takes in a parameter value and returns what Chai calls
// a "chain"
expect(add(2, 4)).to.equal(6);
});
it('ignores additional arguments', function() {
expect(add(2, 4, 6)).to.equal(6);
});
});
function add(a, b) {
return a + b;
}Chai's assertions are popular because they read more like English. You can write expectations like the following:
expect([1, 2]).to.be.an('array').that.does.not.include(3);This syntax can be neat, especially for non-technical readers.
However, Chai syntax is fairly verbose, and can be difficult to write without practice.
We recommend using assert because assert's API is simpler: you can get by with just 6 functions.