Error - Expected 2 Arguments but got 0

Still getting the same error:

  insertData(fname, lname) {
    const body = JSON.stringify({'firstName': fname, 'lastName': lname});
    return this.http.post('http://myurl/insert.php', body, httpOptions);
  }

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>>Still getting the same error:

you're writing something different.

most likely the return part is error

use browser's Inspect tool to verify the exact errors

The errors are pointing to this line:

const body = JSON.stringify({'firstName': fname, 'lastName': lname});

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And only appeared when I added fname and lname to the function

Where do you exactly get the error? in the JSON line, or when calling the function?
In the latter case look where this function is called and if always with two parameters

ref: 'use browser's Inspect tool to verify the exact errors'

I'm using typescript and code won't compile so I won't get to the browser till I can compile it.

Are both fname and lname defined?

so tried exactly what I posted previously?

use var and " instead of const and '

I had to add the parameters to another file. Your change worked fine :)