How to use elasticlunr

folderToDocuments(linksPath)
 .each(result => {
  index.addDoc(result);
 })
 .done(() => {
  const indexJSON = index.toJSON();
  fs.writeFile(indexPath, JSON.stringify(indexJSON), () => {
   console.log('Successfully saved index!');
  });
 });

search(string) {
  return this.index.search(string, {
   expand: true
  });
 }

constructor(basePath) {
  this.basePath = basePath;
  this.index = elasticlunr(function () {
   this.setRef('file');
   this.addField('filename');
   this.addField('content');
  });
  this.files = [];
 }

updateIndexForFile(file) {
  const filePath = path.relative(this.basePath, file);
  this.files.push(filePath);

  return fs.readFile(path.join(this.basePath, filePath), 'utf8')
   .then(content => {
    const doc = {
     file: filePath,
     filename: path.basename(filePath, path.extname(filePath)),
     content
    };
    this.index[this.index.documentStore.hasDoc(filePath) ? 'updateDoc' : 'addDoc'](doc);
    return doc;
   });
 }

getContent(file) {
  return this.index.documentStore.getDoc(file).content;
 }

const run = async () => {
 await folderToDocuments(linksPath, {arrayMod})
  .each(result => index.addDoc(result))
  .toPromise(Promise);
 console.log('Successfully generated index!');