What is the difference between “assert”, “expect”, and “should” in Chai?

Question

What is the difference between assert, expect, and should? When to use what?

assert.         


        

Answer 1

I hope this simple examples makes their differences clear

Assert

var assert = require('chai').assert
const foo = 'bar'
const beverages = { tea: [ 'chai', 'matcha', 'oolong' ] };

assert.typeOf(foo, 'string'); // without optional message
assert.typeOf(foo, 'string', 'foo is a string'); // with optional message
assert.equal(foo, 'bar', 'foo equal `bar`');
assert.lengthOf(foo, 3, 'foo`s value has a length of 3');
assert.lengthOf(beverages.tea, 3, 'beverages has 3 types of tea');

In all cases, the assert style allows you to include an optional message as the last parameter in the assert statement. These will be included in the error messages should your assertion not pass.

Note expect and should uses chainable language to construct assertions, but they differ in the way an assertion is initially constructed. In the case of should, there are also some caveats and additional tools to overcome the caveats.

Expect

var expect = require('chai').expect
const foo = 'bar'
const beverages = { tea: [ 'chai', 'matcha', 'oolong' ] };
expect(foo).to.be.a('string');
expect(foo).to.equal('bar');
expect(foo).to.have.lengthOf(3);
expect(beverages).to.have.property('tea').with.lengthOf(3);

Expect allows you to include arbitrary messages to prepend to any failed assertions that might occur.

var answer = 43;

// AssertionError: expected 43 to equal 42.
expect(answer).to.equal(42);

// AssertionError: topic [answer]: expected 43 to equal 42.
expect(answer, 'topic [answer]').to.equal(42);

This comes in handy when being used with non-descript topics such as booleans or numbers.

Should

The should style allows for the same chainable assertions as the expect interface, however it extends each object with a should property to start your chain. This style has some issues when used with Internet Explorer, so be aware of browser compatibility.

var should = require('chai').should() //actually call the function
const foo = 'bar'
const beverages = { tea: [ 'chai', 'matcha', 'oolong' ] };

foo.should.be.a('string');
foo.should.equal('bar');
foo.should.have.lengthOf(3);
beverages.should.have.property('tea').with.lengthOf(3);

Differences between expect and should

First of all, notice that the expect require is just a reference to the expect function, whereas with the should require, the function is being executed.

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

The expect interface provides a function as a starting point for chaining your language assertions. It works on node.js and in all browsers.

The should interface extends Object.prototype to provide a single getter as the starting point for your language assertions. It works on node.js and in all modern browsers except Internet Explorer.