Q:

\fill left of a node using the positioning library

How can I position a \fill relative to a node? I would position a node in the following way, using the positioning library:
\node (mynode) at (2,4) {text};
\node [left=of mynode] {o};

But if I try the same with a filled circle:
\node (mynode) at (2,4) {text};
\fill [left=of mynode] circle (1);

The circle is centered at origin with no warnings or error messages.

A:

positioning library helps you to position nodes. If you want to use it, you can use a circular node or fix a coordinate node which can be used for further reference.
Here you have both examples. If you use a node be careful because it's minimum size is its diameter but you must also consider its inner sep.
\documentclass[tikz,border=2mm]{standalone}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node[draw] at (2,4) (mynode) {text};
\node [circle, fill=red, left=of mynode, anchor=center, inner sep=0pt, minimum size=6pt] {};

\begin{scope}[yshift=-1cm]
\node[draw] at (2,4) (mynode) {text};
\coordinate [left=of mynode]  (aux);
\fill[green] (aux) circle (3pt);
\end{scope}
\end{tikzpicture}
\end{document}

