Check out example codes for "Property 'value' does not exist on type 'HTMLElement'.". It will help you in understanding the concepts better.

Code Example 1

document.getElementById() returns the type HTMLElement which does not contain a value property.
The subtype HTMLInputElement does however contain the value property.

So a solution is to cast the result of getElementById() to HTMLInputElement like this:

var inputValue = (<HTMLInputElement>document.getElementById(elementId)).value;
<> is the casting operator in typescript.
See TypeScript: casting HTMLElement:

The resulting javascript from the line above looks like this:

inputValue = (document.getElementById(elementId)).value;
i.e. containing no type information.

Learn ReactJs, React Native from