Typescript: Required callback parameters?

Question

I want to define a function which takes a callback as a parameter, and that callback\'s parameters should be required. Typescript correctly reports a callback with a mismatc

Answer 1

There isn't a way to do this.

Callbacks with fewer parameters than the caller provides are extremely common in JavaScript - functions like forEach, map, filter, etc all provide 3 or more arguments but are frequently given 1-parameter functions as callbacks.

Answer 2

You could define a type for the handler. Now you can only pass a callback that has a string argument.

// define a handler so you can use it for callbacks
type Handler = (t: string) => void;

function doSomething(t: string): void {
    console.log("hello " + t)
}

function thisFirst(callback: Handler) {
  callback('world') // ok
  callback(4) // not assignable to type 'string'
}

thisFirst(doSomething)