I think bodytag need to be defined, otherwise it was giving error for me

var bodytag = document.body

Looks like we didn't add "heading" Id earlier to <b> . So, correcting this looks like below:

in html file inside body

<b style="font-size: 40px " id="heading">Hello World!</b>

in Js Console

> hello_world = document.getElementById("heading")

> hello_world.style.fontSize = "100px" //need to explicitly mention px to render the text