Property 'value' does not exist on type 'HTMLElement' for Textarea - Angular

You just have to assert that the type of your element is HTMLTextAreaElement. Because document.getElementById returns HTMLElement and not all html elements have the value property:

let tsnString = (document.getElementById("tsn_list") as HTMLTextAreaElement).value;

But seeing as you use Angular, you should probably be using data binding instead of querying the values yourself

I had faced a similar issue while setting the value of a cell in the table.

I was adding the data as follows:

document.getElementById("table_id").rows[0].cells[0].innerHTML = "data_to_be_added";

And I was getting the following error:

error TS2339: Property 'rows' does not exist on type 'HTMLElement'

Property 'cells' does not exist on type 'HTMLElement'

So I added the following tag and the ERROR got resolved.

(document.getElementById("table_id") as any).rows[0].cells[0].innerHTML = "data_to_be_added";

Here, you are expicitly changing the type of the variable as any.