Discussion on: Understanding Generics in TypeScript

[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)

We can now perform mathematical operations on this value with safety!

[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"

[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"