Extendable Interfaces
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This module introduces you to Interfaces in TypeScript.

Learning Objectives

The objectives of this module are to provide you with an understanding of:

  • How to use Function Types
  • How to use Optionals
  • How to use Excess Property Checks
  • How to use Indexable Types
  • How Interfaces work with Classes
  • How to create Extendable Interfaces

Intended Audience

This learning path is aimed at all who wish to learn how to use TypeScript


It is essential you understand the face of contemporary web development to attend this course. You should have a good working knowledge of Javascript. Please note: Before attending this class delegates must have a Microsoft account (signing up one is free).


We welcome all feedback and suggestions - please contact us at to let us know what you think.


An interface can extend another interface. In the same way a class can extend another class. If an entity signs a contract for the extended interface, all of the requirements of that interface and the interface it extends, must be met. A hasWheels interface extends the vehicle interface, meaning that along with wheels being a required numeric property, any entity that signs that hasWheels contract must have a numeric speed property and may have a Boolean flux capacitor and may have an accelerate method. Classes can implement multiple interfaces giving us fine control over our reusable components. The bus class shown on line 22 implements the hasWheels interface, along with the hasPayingPassengers interface. Values are given to all properties that are required to meet all of the contracts in the constructor and implementations are given for any methods that this class needs to provide. We then create an instance of the class on the right of the screen and use some of its methods logging the results out as we go. The interfaces can be reused across multiple classes. That covers what we need to start using interfaces. Looking at how we can define particular properties and or functions, that objects or classes must implement when we sign that contract. It's included looking at function types, optional properties, access property checks, indexable types, apply an interface to classes and extending interfaces. It'd be worth going getting some practice on that now.


About the Author
Learning Paths

Ed is an Outstanding Trainer in Software Development, with a passion for technology and its uses and holding more than 10 years’ experience.

Previous roles have included being a Delivery Manager, Trainer, ICT teacher, and Head of Department. Ed continues to develop existing and new courses, primarily in web design using: PHP, JavaScript, HTML, CSS, SQL, and OOP (Java), Programming Foundations (Python), and DevOps (Git, CI/CD, etc). Ed describes himself as practically minded, a quick learner, and a problem solver who pays great attention to detail. 

Ed’s specialist area is training in Emerging Technologies, within Web Development. Ed mainly delivers courses in JavaScript covering vanilla JS, ES2015+, TypeScript, Angular, and React (the latter is authored by Ed) and has delivered on behalf of Google for PWAs. Ed has also developed a new suite of PHP courses and has extensive experience with HTML/CSS and MySQL. 

Ed is responsible for delivering QA’s Programming Foundations course using the Eclipse IDE. His skillset extends into the DevOps sphere, where he is able to deliver courses based around Agile/Scrum practices, version control, and CI/CD.

Covered Topics