FullStack Labs

Please Upgrade Your Browser.

Unfortunately, Internet Explorer is an outdated browser and we do not currently support it. To have the best browsing experience, please upgrade to Microsoft Edge, Google Chrome or Safari.
Welcome to FullStack Labs. We use cookies to enable better features on our website. Cookies help us tailor content to your interests and locations and provide many other benefits of the site. For more information, please see our Cookies Policy and Privacy Policy.

What I Discovered About Typescript Generic Types

Written by 
Javier Lopez
Senior Software Engineer
What I Discovered About Typescript Generic Types
blog post background
Recent Posts
From Recommendations to Immersive Try-Ons: AI's Role in Consumer Goods
Mastering Agile Project Management: Navigate Success with a Large Team
Driving Excellence in QA: Continuous Improvement and Postman Best Practices

Table of contents

A few days ago I had a problem in TypeScript where I should have used generic types, but I did not have enough experience with TypeScript. So, as any developer would have done, I searched the web for information about the issue and how I could solve it.

The problem is this: I have an interface with several properties. One of them should be of one type sometimes and of another type other times.

My first approach to solve my issue was this:

interface MyInterface {
    id: number;
    name: string;
    data: MyDataInterface | MySecondDataInterface;

Clearly, this was not the best solution, since when trying to implement this interface the type for data was not correct at the assigning moment. I got the following error: MyDataInterface is not assignable to MyDataInterface | MySecondDataInterface. Definitely a rookie mistake.

The correct solution should be to use a generic type like this:

interface MyInterface<T> {
    id: number;
    name: string;
    data: T;

This way, data could be any type. The implementation could be done creating a type like this: 

type MyFirstType<MyDataInterface>


type MyFirstType<MySecondDataInterface>

Now let’s talk about the subject of the article. Remember, I am a rookie in terms of TypeScript. So when I searched Google for the solution for my initial problem, the term I used was “TypeScript placeholder.” Google was smart enough to return some results that I could use, or at least could be a starting point.

The first result was a GitHub discussion about a new proposal for TypeScript called exactly the same thing: “TypeScript placeholder declaration”. Let’s be clear about something first: the Placeholder Declaration has nothing to do with my initial problem. Generic Types solved my issue.

The Placeholder Declaration proposal covers the occasions when a user needs to express that a type could exist depending on the environment (web or node) in which the code will run. A common example is this:

export declare function printStuff(str: string): void;

 * NOTE: Only works in Node.js
export declare function printStuff(buff: Buffer): void;

Someone that uses a library with this feature would need to say not only that the type Buffer exists but supports some operations. To do so, they could add those operations properly:

declare global {
    interface HTMLElement {
        innerText: string;

export function printStuff(node: HTMLElement) {

What I personally got from reading this whole thread of responses to the proposal was great. It helped me understand many other TypeScript concepts that I did not know until now, and helped me to fall in love a little more with TypeScript. Such a simple problem (as I see it now), such as generic types, led me to this discussion that helped me to understand a lot of things, as well as to follow these people who have a lot to contribute.

The discussion lasted a full year until June 17, 2020, and remains open to this day almost two years later. Unfortunately, it has not concluded and this new functionality has not been added to TypeScript, but it could be added in the future. For more information go to this link.

Javier Lopez
Written by
Javier Lopez
Javier Lopez

I became a developer when a coworker noticed me digging around with Excel macros and convinced me to go deeper. I made the best decision of my life and took up a degree in Technical Information Systems and haven't looked back. I'm prepared to solve problems of any kind, and being a developer gives you the opportunity to face new challenges every day. I've built a JavaScript library for 3D rendering at Autodesk and a tool used by newspapers in the US to produce beautiful data visualizations. I really enjoy working with CSS — with just a few lines of code, you can create something that's structured, maintainable, and easy to understand. I enjoy playing Age of Empies, wathcing anime, exercising, and cooking.

People having a meeting on a glass room.
Join Our Team
We are looking for developers committed to writing the best code and deploying flawless apps in a small team setting.
view careers
Desktop screens shown as slices from a top angle.
Case Studies
It's not only about results, it's also about how we helped our clients get there and achieve their goals.
view case studies
Phone with an app screen on it.
Our Playbook
Our step-by-step process for designing, developing, and maintaining exceptional custom software solutions.
VIEW OUR playbook
FullStack Labs Icon

Let's Talk!

We’d love to learn more about your project.
Engagements start at $75,000.

company name
Type of project
How did you hear about us?
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.