The ultimate guide to coding interviews: Learn and practice JavaScript design patterns tested by FAANG engineers. Get interview-ready in just a few hours with battle-tested solutions.

If you're an experienced developer, chances are your interviewer won't ask you about language syntax. Instead, they'll focus on things that test your understanding of software architecture, and how capable you are at finding optimized solutions to given problems. 

That’s where a strong understanding of design patterns comes in handy. It's often useful to be able to draw on established patterns instead of reinventing the wheel.

In this course, you will learn how to effectively deploy design patterns to answer interview questions. The course is divided into four sections: Creational Patterns, Structural Patterns, Behavioral Patterns, and Architectural Patterns. Each section has its own set of design patterns which you’ll learn to use through various challenges. 

Every challenge contains a detailed solution explanation. By the end of this course (and with some practice), you will be able to confidently use design patterns to tackle advanced coding interview questions in JavaScript.

JavaScript Design Patterns for Coding Interviews

## Explanation

The task was to create a function `createToy`, which instantiated either a `ToyDuck` or a `ToyCar` object. 

We start by defining the constructor functions. Let's take a look at them.

* **`ToyDuck`**

  ```
  function ToyDuck(toyObj){
    this.color = toyObj.color;
    this.price = toyObj.price;
  }
  ```

  It accepts a parameter `toyObj`, that is, the "toy object", sets its `color` equal to `toyObj.color` and its `price` equal to `toyObj.price`.

* **`ToyCar`** 

  ```
  function ToyCar(toyObj){
    this.color = toyObj.color;
    this.price = toyObj.price;
    this.name =  toyObj.name;
  } 
  ```

  It accepts a parameter `toyObj`, that is, the "toy object" and sets its `color` equal to `toyObj.color`, a `price` equal to `toyObj.price` and a `name` equal to `toyObj.name`.

Now that we have defined our constructor functions, we implement the factory pattern by implementing the `createToy` method. 

We define this method inside the `ToyFactory` constructor function. Let's look at it in detail:

```
function ToyFactory() {
    this.toy = ToyDuck;
    this.createToy = function(toyChosen) {
        if (toyChosen.toyType == "duck") {
            this.toy = ToyDuck;
        } else if (toyChosen.toyType == "car") {
            this.toy = ToyCar;
        }
        return new this.toy(toyChosen);
    }
}
```  

First, we have the `toy` value set to the `ToyDuck` constructor function (for ease, let's refer to these as "class"). What we want is to create either a duck or a car toy, depending on the requirement. 

The `createToy` method accepts a parameter `toyChosen`. Next, it checks whether the type of the `toyChosen` is `duck` or `car`.  You can see this on **lines 4 & 6**.

```
if (toyChosen.toyType == "duck") //line 4
else if (toyChosen.toyType == "car") //line 6
```

The `toyType` property stores the type of the toy, i.e., `duck` or `car`. Depending on its value, it sets the value of `toy` equal to either `ToyDuck` class or `ToyCar` class. You can see this on **lines 5 & 7**.

```
this.toy = ToyDuck //line 5
this.toy = ToyCar //line 7
```

In the end, the function instantiates an object of the required class (depending on the class stored in `toy`) and returns the object. You can see this on **line 9**.

```
return new this.toy(toyChosen); //line 9
```

Let’s run our implementation for the following example:


```
var toyFactory = new ToyFactory();
var car = toyFactory.createToy({
    toyType: "car",
    color: "blue",
    price: 12,
    name: "honda"
})
```

* First, we instantiate an object of the `ToyFactory` class called `toyFactory`.

* Next, we invoke the `createToy` function and pass an object as a parameter to it.

* In the `createToy` function, the line `toyChosen.toyType == "car"` will evaluate to **true**. Hence, `toy` will be set to the `ToyCar` class.

* In the end, an object of the `ToyCar` class is instantiated and returned.


Therefore, the toy objects aren’t instantiated with `new` directly; instead, you directly call `createToy` which handles the object creation.


# Explanation

The task was to create a function `createToy`, which instantiated either a `ToyDuck` or a `ToyCar` object. 

We start by defining the constructor functions. Let's take a look at them.

* **`ToyDuck`**

  ```
  function ToyDuck(toyObj){
    this.color = toyObj.color;
    this.price = toyObj.price;
  }
  ```

  It accepts a parameter `toyObj`, that is, the "toy object", sets its `color` equal to `toyObj.color` and its `price` equal to `toyObj.price`.

* **`ToyCar`** 

  ```
  function ToyCar(toyObj){
    this.color = toyObj.color;
    this.price = toyObj.price;
    this.name =  toyObj.name;
  } 
  ```

  It accepts a parameter `toyObj`, that is, the "toy object" and sets its `color` equal to `toyObj.color`, a `price` equal to `toyObj.price` and a `name` equal to `toyObj.name`.

Now that we have defined our constructor functions, we implement the factory pattern by implementing the `createToy` method. 

We define this method inside the `ToyFactory` constructor function. Let's look at it in detail:

```
function ToyFactory() {
    this.toy = ToyDuck;
    this.createToy = function(toyChosen) {
        if (toyChosen.toyType == "duck") {
            this.toy = ToyDuck;
        } else if (toyChosen.toyType == "car") {
            this.toy = ToyCar;
        }
        return new this.toy(toyChosen);
    }
}
```  

First, we have the `toy` value set to the `ToyDuck` constructor function (for ease, let's refer to these as "class"). What we want is to create either a duck or a car toy, depending on the requirement. 

The `createToy` method accepts a parameter `toyChosen`. Next, it checks whether the type of the `toyChosen` is `duck` or `car`.  You can see this on **lines 4 & 6**.

```
if (toyChosen.toyType == "duck") //line 4
else if (toyChosen.toyType == "car") //line 6
```

The `toyType` property stores the type of the toy, i.e., `duck` or `car`. Depending on its value, it sets the value of `toy` equal to either `ToyDuck` class or `ToyCar` class. You can see this on **lines 5 & 7**.

```
this.toy = ToyDuck //line 5
this.toy = ToyCar //line 7
```

In the end, the function instantiates an object of the required class (depending on the class stored in `toy`) and returns the object. You can see this on **line 9**.

```
return new this.toy(toyChosen); //line 9
```

Let’s run our implementation for the following example:


```
var toyFactory = new ToyFactory();
var car = toyFactory.createToy({
    toyType: "car",
    color: "blue",
    price: 12,
    name: "honda"
})
```

* First, we instantiate an object of the `ToyFactory` class called `toyFactory`.

* Next, we invoke the `createToy` function and pass an object as a parameter to it.

* In the `createToy` function, the line `toyChosen.toyType == "car"` will evaluate to **true**. Hence, `toy` will be set to the `ToyCar` class.

* In the end, an object of the `ToyCar` class is instantiated and returned.


Therefore, the toy objects aren’t instantiated with `new` directly; instead, you directly call `createToy` which handles the object creation.


This lesson will explain the solution to the problem from the previous coding challenge.

Before We Begin

Introduction To Design Patterns

Creational Patterns

Structural Patterns

Behavioral Patterns

Architectural Patterns

Conclusion

Challenge: Solution Review

Solution #

Explanation