How to synchronize an asynchronous function?

Question

JavaScript

Hello, I'm minifying css with css-nano, however it can only work in asynchronous mode.
An example of a minimize function using css-nano

const postcss = require("postcss");
const cssnano = require("cssnano");
const cssNanoPresetAdvanced = require("cssnano-preset-advanced");

let MINIFY_CSS = function (code) {
          postcss(cssnano({ preset: cssNanoPresetAdvanced }))
            .process(code)
            .then(function (result) {
              console.log(result.css);
            });
        },

you can use for example However, the problems start when I try to return a value, and not just output to the console. This is necessary for the code chains to work, for example
MINIFY_CSS("html{background:green;}")

EXTRACT_CSS(MINIFY_CSS(require("!!raw-loader!sass-loader!./MyScss.scss")))

My attempt is in the form

let MINIFY_CSS= function (code) {
          let minified;
          async () => {
            await postcss(cssnano({ preset: cssNanoPresetAdvanced }))
              .process(code)
              .then(function (result) {
                minified = result.css;
              });
          };
          console.log(minified);
          return minified;
        },

It prints undefined, which means that the value is returned before the async function completes and nothing works...
How can I improve my code or synchronize the async function?