How can I get an element with getElementById found in innerHTML

I want to do something like this:

<div id="parent">
  <iframe id="myFrame" title="HEY!" srcdoc="<div id='inner'>Hello World!</div>"></iframe>
</div>
var parent = document.getElementById("parent").innerHTML;
var title = parent.getElementById("myFrame").title;

But this throws an error like innerHTML doesn't have getElementById attribute.
How can I get the title of the iframe element in the innerHTML.
Note: It must be inside the innerHTML element.

First of all, the .innerHTML returns a string, not a DOM element. So as many suggested, you should remove this.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

var parent = document.getElementById("parent");
var title = parent.getElementById("myFrame").title;

<!-- language: lang-html -->

<div id="parent">
  <iframe id="myFrame" title="HEY!" srcdoc="<div id='inner'>Hello World!</div>"></iframe>
</div>

<!-- end snippet -->

BUT THIS NOT ONLY OUTPUTS AN ERROR, BUT IT ALSO DOESN'T WORK !

It would for getElementByClassName() and getElementByTagName(), but that's not the point...

You are running the getElementById() method on an element, and this doesn't work : why would you run this method scoped ? what's the point as IDs have to be unique (but not classes or tags) ?

So here, this is enough :

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

var title = document.getElementById("myFrame").title;
console.log(title);

<!-- language: lang-html -->

<div id="parent">
  <iframe id="myFrame" title="HEY!" srcdoc="<div id='inner'>Hello World!</div>"></iframe>
</div>

<!-- end snippet -->

EDIT :
Just to make things clear :

You can only call getElementByID() on document as discussed here

Try this,

var parent = document.getElementById("parent");
var title = parent.querySelector("#myFrame").title;