Modify from: https://github.com/astyle-format/vscode-format/blob/master/src/extension.js

format-astyle-extension.js

const vscode = require('vscode'),
  childProcess = require('child_process'),
  fs = require('fs');

// Global vars

// Status bar
const statusBar = vscode.window.createStatusBarItem(vscode.StatusBarAlignment.Left);

// Show message in StatusBar
function checkStatusBar(text, delay) {
  statusBar.text = text;
  statusBar.show();
  setTimeout(() => statusBar.hide(), delay);
}

function activate(context) {
  const disposable = vscode.commands.registerCommand('extension.aformat', function () {
    // Get current editor
    const activeTextEditor = vscode.window.activeTextEditor,
      // Current document
      document = activeTextEditor.document,
      // Path to current file
      currentFilePath = document.fileName,
      // Get extension configuration
      configuration = vscode.workspace.getConfiguration('astyleFormat'),
      // Config: show message box about formatting
      showMessages = configuration['show-message'],
      // Where is .astylerc file
      astylerc = configuration.astylerc,
      // Where is astyle program
      astyle = configuration.astyle || "astyle",
      // Timeout for message box
      delay = configuration.messageTimeout,
      // Format function
      aformatFunc = () => {
        let args = [];
        args.push(astyle);
        if (astylerc) {
          args.push("--options=\"" + astylerc + "\"");
        }
        args.push("\"" + currentFilePath + "\"");

        let defaultMaxBufferSize = 200 * 1024;
        let maxBufferMultiplierSize = fs.statSync(currentFilePath).size * 2;
        // Assume that the formatting/expansion of the document could double it's size
        // Make sure the minimum size is still the default for execFile::maxBuffer
        let maxBufferSize = Math.max(defaultMaxBufferSize, maxBufferMultiplierSize);

        childProcess.exec(args.join(' '), {maxBuffer: maxBufferSize}, (error, out, err) => {
          checkStatusBar('$(pencil) Formatted', delay);

          if (showMessages) {
            error ? vscode.window.showErrorMessage('Some error while formatting: (reason: "' + err.split(/\r\n|\r|\n/g).join(',') + ' ' + error + '")')
                  : vscode.window.showInformationMessage('Formatted!');
          }
        });
      };
    // If document is editing now
    if (document.isDirty) {
      // Then save the document and make format
      document.save().then(() => {
        aformatFunc();
      });
    } else {
      // Else just formatting
      aformatFunc();
    }
  });
  context.subscriptions.push(disposable);
}
exports.activate = activate;

// this method is called when your extension is deactivated
function deactivate() {}

exports.deactivate = deactivate;