Here is working Example

onFileChange(ev) {

    let workBook = null;

    let jsonData = null;

    const reader = new FileReader();

    const file = ev.target.files[0];

    reader.onload = (event) => {

      const data = reader.result;

      workBook = XLSX.read(data, { type: 'binary' });

      jsonData = workBook.SheetNames.reduce((initial, name) => {

        const sheet = workBook.Sheets[name];

        initial[name] = XLSX.utils.sheet_to_json(sheet);

        return initial;

      }, {});

      const dataString = JSON.stringify(jsonData);

      document.getElementById('output').innerHTML = dataString.slice(0, 300).concat(...);

      this.setDownload(dataString);

    }

    reader.readAsBinaryString(file);

  }