How to render while in the compositor? In a tutorial at 3:22 the guy rendered while in node editor, but didn't say how.

I have tried other tutorials but they all don't say how to do this.
At 3:22 he renders the scene by pressing F12.

You can render by


Pressing F12.
Pressing Render image in Info header > Render:


Pressing Render in Properties > Render settings > Render: