When using Self-Invoking Functions I got the following error:

Uncaught TypeError: document.write(...) is not a function at index.html:86:29

the entire code:

document.write('<br />')
      //second soloution:
      const anonymousFunc3 = function (x, y) {
        if (x === 'undefined') {
          x = 0 // assign a value to x in case that 'x' is "undefined"
        }
        if (y === 'undefined') {
          y = 0
        }
        return x + y
      }
      document.write(anonymousFunc3())
      document.write('<br />')
      //5
      //this way, the function is executed itself, without any call!

      (function () {
        document.write('Ok!')
        //alert('')
      }
    )()

And here I'm adding a picture of the browser to more clear the problem:

Could anyone explain it to me?

I also found that if I place the error-pane code within another <script> tag, the error will be gone! Please take a look at the following codes to more understand:

<script>
.
.
.
</script>
<script>
 (function () {
        document.write('Ok!')
        //alert('')
 })()
</script>

Till now, I don't know the why?