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

# Problem statement 

In this challenge, you have to implement a part of an online hair product ordering system. The available products are `shampoo`, `conditioner`, and `hair serum`. 
A customer who is shopping online can buy any of these. A `product` object has the following properties:

* `productName`: name of the product, that is, `shampoo`, `conditioner`, or `hair serum`

* `amount`: the number of bottles that the customer wants to buy

This system will allow a customer to buy an `amount` of product. If that `amount` is available in the inventory, a `BuyProduct` class instance should be initiated. If the `amount` is not available, a `PreOrderProduct` class instance should be initiated. The inventory will look like:

* the amount of `shampoo` is: `20`
* the amount of `conditioner` is: `20`
* the amount of `hair serum` is: `1000`


In the end, the customers should get a message that lets them know if they can buy that `amount` of bottles or will have to pre-order. Here’s an example:


```
`2 bottles of shampoo are available. Click on "buy" to purchase them.`

`2000 bottles of hair serum are not available. You can Pre-order them on the next page.` 
```

You need to implement the facade pattern to achieve this. You've already been given the `Inventory`, `BuyingProduct`, `BuyProduct`, and `PreOrder` classes. Write their definitions and link them such that the output mentioned above is shown to the customer.



# Input

`buyProduct` method invoked with some given arguments

# Output

The message displayed regarding whether the customer can buy the products or will have to pre-order them

# Sample input

```
var customer = new BuyingProduct()
customer.buyProduct({productName: "shampoo", amount : 2})
customer.buyProduct({productName: "hair serum", amount : 2000})
```

# Sample output

```
`2 bottles of shampoo are available. Click on "buy" to purchase them.`

`2000 bottles of hair serum are not available. You can Pre-order them on the next page.`
```

In this challenge, you have to implement the facade pattern to solve the given problem.

Before We Begin

Introduction To Design Patterns

Creational Patterns

Structural Patterns

Behavioral Patterns

Architectural Patterns

Conclusion

Challenge: Facade Pattern

Problem statement