How to run a function inside of a function in JavaScript

To keep with your current usage, try this:

function myFunction()
{
    return {
        play: function(a, b)
        {
            document.getElementById("demo").innerHTML = a * b;
        }
    }
}

myFunction().play(2,3);

So what you've done with myFunction is nested a 'private' function play inside so the outside cannot call it. So you can either return the inner function or call play inside of function like and only call myFunction:

function myFunction(a, b) {
    function play(a, b) {
        document.getElementById("demo").innerHTML = a * b;
    }
    play(a, b);
}
myFunction(2,3); // will set innerHTML of demo to a*b

Or remove the nesting and just have

function play(a, b) {
  document.getElementById("demo").innerHTML = a * b;
}
play(2,3);

You will not be able to access the play function where you are currently invoking. The play function is out of scope.

function myFunction()
{
    function play(a, b)
    {
        document.getElementById("demo").innerHTML = a * b;
    }

    play(2,3);
}

myFunction()

This should work for you

You can't do this because that function play only exists within the function myFunction and everything outside of it won't be able to execute it.

An alternative is to return an object with a property called play whose value is the function you need to execute.

function myFunction() {
  return {
    play: function(a, b) {
      document.getElementById("demo").innerHTML = a * b;
    }
  }
}

myFunction().play(2, 3);

<p id="demo"></p>