Property 'value' does not exist on type EventTarget in TypeScript

event.target here is an HTMLElement which is the parent of all HTML elements, but isn't guaranteed to have the property value. TypeScript detects this and throws the error. Cast event.target to the appropriate HTML element to ensure it is HTMLInputElement which does have a value property:

(event.target as HTMLInputElement).value

Per the documentation:

Type the $event

The example above casts the $event as an any type. That simplifies the code at a cost. There is no type information that could reveal properties of the event object and prevent silly mistakes.

[...]

The $event is now a specific KeyboardEvent. Not all elements have a value property so it casts target to an input element.

(Emphasis mine)

Here's another fix that works for me:

(event.target as HTMLInputElement).value

That should get rid of the error by letting TS know that event.target is an HTMLInputElement, which inherently has a value. Before specifying, TS likely only knew that event alone was an HTMLInputElement, thus according to TS the keyed-in target was some randomly mapped value that could be anything.

Passing HTMLInputElement as a generic to the event type should work too:

onUpdatingServerName(event: React.ChangeEvent<HTMLInputElement>) {
  console.log(event);
  this.newserverName = event.target.value;
}