Are strongly typed functions as parameters possible in TypeScript?

TLDR

This question does not need to be overly explained. The short answer is yes.

Functions can be strongly typed — even as parameters to other functions.

How to use strongly typed function parameters

The accepted answer on this stack overflow post is correct — to a degree.

Assuming you had a function: speak:

function speak(callback) {
  const sentence = "Hello world"
  alert(callback(sentence))
}

It receives a callback that’s internally invoked with a string.

To type this, go ahead and represent the callback with a function type alias:

type Callback = (value: string) => void

And type the speak function as follows:

function speak(callback: Callback) {
  const sentence = "Hello world"
  alert(callback(sentence))
}

Alternatively, you could also keep the type inline:

function speak(callback: (value: string) => void) {
  const sentence = "Hello world"

  alert(callback(sentence))
}

See Typescript playground.

And there it is!

You’ve used a strongly typed function as a parameter.

Functions that return any

The accepted answer in the referenced stack overflow post for example says the callback parameter's type must be "function that accepts a number and returns type any"

That’s partly true, but the return type does NOT have to be any

In fact, do NOT use any.

If your function returns a value, go ahead and type it appropriately:

// Callback returns an object
type Callback = (value: string) => { result: string }

If your callback returns nothing, use void not any:

// Callback returns nothing
type Callback = (value: string) => void

Note that the signature of your function type should be:

(arg1: Arg1type, arg2: Arg2type) => ReturnType

Where Arg1type represents the type of the argument arg1, Arg2type the type of the arg2 argument, and ReturnType the return type of your function.

Conclusion

Functions are the primary means of passing data around in Javascript.

Typescript not only allows you to specify the input and output of functions, but you can also type functions as arguments to other functions.

Go ahead and use them with confidence.

Get a Free Typescript Book?

Get this book for free