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Discussion on: Understanding Generics in TypeScript

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plondon profile image
Philip London Author

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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iam_danieljohns profile image
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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plondon profile image
Philip London Author

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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