Syntax error in LuaLaTeX \directlua code

I have the below document that I try to compile with LuaLaTex (using the MiKTeX package).

\documentclass{article}

\usepackage{pgfplots}

\pgfplotsset{compat=newest}

\begin{document}
\expandafter\pgfplotstableread\directlua {
    global json = utilities.json.load("test.json")
    tex.sprint("{\string\r")
    tex.sprint("x y\string\r")
    for n, row in ipairs(json.data) do
        tex.sprint(row[1] .. " " .. row[2] .. "\string\r")
    end
    tex.sprint("}")
}\loadedtable

\begin{tikzpicture}
  \begin{axis}
    \addplot table {\loadedtable};
  \end{axis}
\end{tikzpicture}
\end{document}

but I get a compilation error due to a Syntax error in the Lua part:

[\directlua]:1: syntax error near 'json'.
l.17 }
  \loadedtable

I'm a noob at LaTeX and unfortunately I have no clue of Lua either, but from what I have researched I can't figure out where there is a syntax error.

The respective test.json is located in the same folder like the .tex script and has this form:

{
    "name":"list_of_xy_values",
    "data":[[1,2],[2,4],[3,6],[4,3],[5,0]]
}

This question refers to this other question