How to use the node-elm-compiler.compileToString function in node-elm-compiler

var intervalId = setInterval(function(){
    if (runningInstances >= maxInstances) return;
    runningInstances += 1;
    clearInterval(intervalId);

    // If we are running in watch mode, and we have previously compiled
    // the current file, then let the user know that elm-make is running
    // and can be slow
    if (alreadyCompiledFiles.indexOf(resourcePath) > -1){
      console.log('Started compiling Elm..');
    }

    var compilation = elmCompiler.compileToString(files, options)
      .then(function(v) { runningInstances -= 1; return { kind: 'success', result: v }; })
      .catch(function(v) { runningInstances -= 1; return { kind: 'error', error: v }; });

    promises.push(compilation);

    Promise.all(promises)
      .then(function(results) {
        var output = results[results.length - 1]; // compilation output is always last

        if (output.kind == 'success') {
          alreadyCompiledFiles.push(resourcePath);
          callback(null, output.result);
        } else {
          output.error.message = 'Compiler process exited with error ' + output.error.message;
          callback(output.error);
        }

build() {
    let options = Object.assign({}, this.options);
    let files = this.listFiles();
    if (!files.length) {
      // Nothing to build
      return;
    }

    return compileToString(files, options)
      .then(data => {
        // elm-make output
        let jsStr = data.toString();

        // fix module exports
        jsStr =
          `
          var elmScope = {};
          (function() {
            ${jsStr}
          }).call(elmScope)
          export default elmScope.Elm;
          `;

        // build
        const dir = path.join(this.outputPath, this.destDir);

var compile = function (filename) {
  return compiler.compileToString([filename], {yes: true, cwd: fixturesDir})
    .then(function (data) {
      return [data.toString(), 'module.exports = Elm;'].join('\n');
    });
}

var compile = function (filename) {
  return compiler.compileToString([filename], {cwd: fixturesDir})
    .then(function (data) {
      return data.toString();
    });
};

return new Promise((resolve, reject) => {
    NodeElmCompiler.compileToString(...args)
    .then(data => resolve({ type: 'ok', data }))
    .catch(data => resolve({ type: 'error', data }))
  })
}