Setting up node-sass directly is needlessly complicated and the documentation sucks.

node-sass.js

const sass = require('node-sass');
const fs = require('fs');

const scssFile = 'src/site/_includes/scss/style.scss';
const cssFile = 'src/css/style.css';

sass.render({
  file: scssFile,
  sourceMap: true,
  outputStyle: 'compressed',
  outFile: cssFile,
  // importer: function (url, prev, done) {


  //   someAsyncFunction(url, prev, function (result) {
  //     done({
  //       file: result.path, // only one of them is required, see section Special Behaviours.
  //     });
  //   });
  //   // OR
  //   var result = someSyncFunction(url, prev);
  //   return {
  //     file: result.path,
  //     contents: result.data
  //   };
  // }



}, function (error, result) { // node-style callback from v3.0.0 onwards
  if (error) {
    console.log(error.status); // used to be "code" in v2x and below
    console.log(error.column);
    console.log(error.message);
    console.log(error.line);
  } else {
    console.log(`\n#########################\nCSS output: \n${result.css.toString()}\n#########################\n`);
    console.log(result.stats);
    // console.log(result);
    // console.log(result.map.toString());
    // console.log(JSON.stringify(result.map));

    fs.writeFile(cssFile, result.css, function (err) {
      if (!err) {
        // console.log('OK it worked?')
      }
    });
  }
});