Indexable Types
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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).


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Similar to interfaces that describe function types, we can describe types we can index into. These indexable types have an index signature that describes the types we can use to index the object. Along with the corresponding types returned when indexing. The GarageArray1 interface specifies that each number index must have a string value. We define an array to implement a GarageArray1 interface. We can then access the string by using the array name and the index number. The GarageArray2 interface specifies that each index is a string and the value is a number. We define an array to implement the GarageArray2 to interface we can then access the number using the key. You can make index signatures read only to stop them being mutated. The third GarageArray demonstrates this with the interphase specifying read only and the error message when trying to reassign a key value.


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.

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