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Understanding Generics in TypeScript

Philip London on November 10, 2021

Quick note! If you'd like to experience this post interactively, go to https://codeamigo.dev/lessons/151 Introduction Sometimes when ...
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Mike Ekkel •

This is a great explanation but I'm still unsure of a situation where I would use generics.

function returnStringOrNumber(arg: string | number) { return arg }
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The above example can have multiple return types without the need of a generic.

function returnStringOrNumber(arg: string | number): string | number { return arg }
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So I'm not sure how a generic would add to that. Granted, that's mostly due to my lack of understanding of generics in the first place 😄

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Philip London •

Hey Mike! Did you checkout the interactive tutorial? Maybe that would be helpful as well!

To answer your question, let's say we used returnStringOrNumber and assigned a variable to it.

const myStringOrNum = returnStringOrNumber(123)

Now try performing a mathematical function on myStringOrNum! TypeScript doesn't know if that's valid or not because as we said, returnStringOrNumber might return a string!

However, if we used generics:

function returnStringOrNumber<T>(arg: T): T { return arg } const myStringOrNum = returnStringOrNumber<number>(123)
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We can now perform mathematical operations on this value with safety!

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Daniel Johns •

I tried this and it still ran. Is this just a JS issue or am I missing something?

function returnStringOrNumber<T>(arg: T): T { return arg } let value = returnStringOrNumber<number>(123) + "hello"
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Philip London •

Hey @iam_danieljohns that's perfectly valid JavaScript, in this case value will be cast to a string type because concatenating a number and a string results in a string. In this case value will be "123hello".

If you wanted to make sure that value was indeed a number type you could do:

let value: number = returnStringOrNumber<number>(123) + "hello"
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OldMan Montoya •

Think consuming an API, you can get different types of responses, here generics are gold , because you create the function to consume once using generics and you just specify the type when you use it

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Mike Ekkel •

That's a very good point :)

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yossarian •

Most of the time you need to use generics when one argument depends on another catchts.com/infer-arguments or you need to do some validation catchts.com/type-negation , catchts.com/validators

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Mike Ekkel •

Thanks for the resources!

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Bruno •

Simple and objetive description abou how generic works in typescript.
Very good! I will recommend it.

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Matyáš Teplý •

I'm learning TypeScript right now, and this was a really simple, digestible way of explaining the topic, thank you!

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Damien • • Edited

Good reading! I think that instead of a ThingWithLength you could simply replace that with an Array just in case someone would expect more array methods from it :)