Can you explain the variations between assert, expect, and should? How do you know when to utilize each one?
assert.equal(3, '3', '== turns values into strings');
var foo = 'bar';
expect(foo).to.equal('bar');
foo.should.equal('bar');
Visit this link for detailed documentation on the distinctions between the three interfaces.
Each interface offers a unique approach to performing assertions, though they all achieve the same end goal. Some users may have a preference for one style over the others. Additionally, there are some technical nuances worth noting:
The assert and expect interfaces do not alter Object.prototype, unlike the should interface. This makes them a better choice in environments where modifying Object.prototype is not desirable or possible.
Custom messages are widely supported by the assert and expect interfaces. For example:
assert.isTrue(foo, "foo should be true");
expect(foo, "foo should be true").to.be.true;
If an assertion fails, the message "foo should be true" will be displayed alongside the error with assert and expect, but not with should.
In previous versions, it was believed that custom messages in expect required a workaround. However, this is no longer necessary as passing a message as a second parameter works seamlessly now.
Note that without a custom message, assert.isTrue(foo), expect(foo).to.be.true, and foo.should.be.true all yield the same generic error message if foo === 1:
AssertionError: expected 1 to be true
Therefore, while expect and should may offer better readability, none of the interfaces provide specific details about what exactly was being tested when an assertion fails. Adding a custom message is crucial for clarity in such cases.
I believe this straightforward illustration effectively clarifies their distinctions
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');
For all scenarios, the assert style allows you to add an optional message as the final parameter in the assert statement. These messages will be displayed in error messages if your assertion fails.
Note
expect and should use chainable language for creating assertions, but they differ in how an assertion is first established. With should, there are also some limitations and additional tools available to address these shortcomings.
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 enables you to include customized messages before any failed assertions 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 feature is useful when dealing with vague topics such as booleans or numbers.
Should
The should style offers the same chainable assertions as the expect interface, but it augments each object with a should property to initiate the chain. However, this method may encounter compatibility issues when used in Internet Explorer.
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
Initially, notice that the expect require simply references the expect function, while with the should require, the function is executed.
var chai = require('chai')
const expect = chai.expect
const should = chai.should();
The expect interface provides a function to start chaining your language assertions. It functions on node.js and all browsers.
The should interface enhances Object.prototype to offer a single getter as the starting point for your language assertions. It works on node.js and all modern browsers except Internet Explorer.
From my understanding, there is a significant distinction between an assert and an expect in terms of their impact on code execution. When an assert fails, it halts the program immediately. However, when an expect statement fails, it does not stop the program.
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