Pattern for ensuring testability in JavaScript

Question

Pattern for ensuring testability in JavaScript

I have come across two different JavaScript testable patterns, but I am struggling to understand the distinctions between them and which one would be more suitable for my needs.

Pattern 1:

function MyClass () {
    this.propertyA = null;
    this.methodA = function() {
        alert('');
    }
}

var object = new MyClass();
object.methodA();

Pattern 2:

var object = (function() {
    var propertyA = null;
    return {
        methodA: function() {
            alert('');
        }
    }
}());
var object = new MyClass();
object.methodA();

What sets these patterns apart? Which one is more suitable for unit testing using jQuery's QUnit? It's worth noting that I cannot utilize jQuery plugins due to specific requirements focused on business logic (no UI testing). Thank you for any guidance you can provide.

javascript

Answer 1

Answer №1

When it comes to QUnit, its main purpose is to provide a neat and organized way to display assertions during testing.

Regarding the comparison between the two coding patterns, Pattern 1 that you have implemented involves all-public properties. However, keep in mind that private properties are also possible:

function MyClass () {
    var propertyA = null;
  
    this.methodA = function() {
        alert('');
    }
}

var obj = new MyClass();
obj.methodA();

On the other hand, Pattern 2 may not function as expected. By enclosing the result object in a closure, using new on it won't work properly. To make it functional, consider structuring it like this:

var MyClass = (function() {
    var propertyA = null;

    return function () {
        return {
            methodA: function() {
                alert('');
            }
        };
    }
}());
var obj = new MyClass();
obj.methodA();

The essence of the new keyword is that it activates the object's constructor with a fresh variable tied to this. Without a proper constructor, you're essentially generating a singleton object.

To summarize the actual behavior:

In Pattern 1, each instance of MyClass has its own unique propertyA.
In your implementation of Pattern 2, a single shared object is created.
In my version of Pattern 2, all instances share the same propertyA (similar to static members in C++).

This distinction occurs because Pattern 1 establishes a straightforward constructor function creating separate closures for public methods upon new invocation. Conversely, Pattern 2 forms a sole closure around the constructor and declares variables external to the constructor's scope.

Answer 2

When it comes to QUnit, its main purpose is to provide a neat and organized way to display assertions during testing.

Regarding the comparison between the two coding patterns, Pattern 1 that you have implemented involves all-public properties. However, keep in mind that private properties are also possible:

function MyClass () {
    var propertyA = null;
  
    this.methodA = function() {
        alert('');
    }
}

var obj = new MyClass();
obj.methodA();

On the other hand, Pattern 2 may not function as expected. By enclosing the result object in a closure, using new on it won't work properly. To make it functional, consider structuring it like this:

var MyClass = (function() {
    var propertyA = null;

    return function () {
        return {
            methodA: function() {
                alert('');
            }
        };
    }
}());
var obj = new MyClass();
obj.methodA();

The essence of the new keyword is that it activates the object's constructor with a fresh variable tied to this. Without a proper constructor, you're essentially generating a singleton object.

To summarize the actual behavior:

In Pattern 1, each instance of MyClass has its own unique propertyA.
In your implementation of Pattern 2, a single shared object is created.
In my version of Pattern 2, all instances share the same propertyA (similar to static members in C++).

This distinction occurs because Pattern 1 establishes a straightforward constructor function creating separate closures for public methods upon new invocation. Conversely, Pattern 2 forms a sole closure around the constructor and declares variables external to the constructor's scope.

Answer 3

Answer №2

The distinction between these approaches lies in the way the second pattern conceals internal properties - after creating an object, propertyA cannot be accessed directly; only methodA can be called (which in turn has access to propertyA). This makes pattern 2 superior from a standpoint of object-oriented encapsulation.

When it comes to testing, however, the difference should not be noticeable since tests should be based on the "public" API of the object. The methods tested should align with those that an actual user of the object would employ, making both patterns functionally identical. There might be rare instances where accessing propertyA from outside is necessary for testing purposes, indicating a potential design flaw. If redesigning to prevent such scenarios is not feasible, then pattern 1 would have to be utilized.

Answer 4

The distinction between these approaches lies in the way the second pattern conceals internal properties - after creating an object, propertyA cannot be accessed directly; only methodA can be called (which in turn has access to propertyA). This makes pattern 2 superior from a standpoint of object-oriented encapsulation.

When it comes to testing, however, the difference should not be noticeable since tests should be based on the "public" API of the object. The methods tested should align with those that an actual user of the object would employ, making both patterns functionally identical. There might be rare instances where accessing propertyA from outside is necessary for testing purposes, indicating a potential design flaw. If redesigning to prevent such scenarios is not feasible, then pattern 1 would have to be utilized.

Pattern for ensuring testability in JavaScript

Answer №1

Answer №2

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