Upload JSON file using Angular 6

But when I'm trying to print it's content using JSON.stringify I get: {} (empty file).

This is not a content of JSON file. It's a File object. To read content of JSON you need to use FileReader

onFileChanged(event) {
  this.selectedFile = event.target.files[0];
  const fileReader = new FileReader();
  fileReader.readAsText(this.selectedFile, "UTF-8");
  fileReader.onload = () => {
   console.log(JSON.parse(fileReader.result));
  }
  fileReader.onerror = (error) => {
    console.log(error);
  }
}