javascript - Upload a File and Read Data with FileReader in Angular 2

Question

I am trying to create an upload form in Angular 2 ts (2.2.1), that allows the upload of e.g. a CSV file, but instead of the file being sent straight to a http service I want the file first to be validated in the browser.

So far I can already upload a file and print it in the console with this code:

1. Html input for file upload

   <input type="file" (change)="changeListener($event)" #input />
   

2. In my angular component I have set up the eventListner and the File reader.

   export class UploadComponent {
   
       public fileString;
   
       constructor() {
           this.fileString;
       }
   
       changeListener($event): void {
               this.readThis($event.target);
           }
   
       readThis(inputValue: any): void {
           var file: File = inputValue.files[0];
           var myReader: FileReader = new FileReader();
           var fileType = inputValue.parentElement.id;
           myReader.onloadend = function (e) {
               //myReader.result is a String of the uploaded file
               console.log(myReader.result);
   
               //fileString = myReader.result would not work, 
               //because it is not in the scope of the callback
           }
   
           myReader.readAsText(file);
       }
   }
   

This code works perfectly fine so far.

However I have not found a way to store the data from the reader in a way that allows me to access it with my angular component.

The myReader.onloadend() callback function does not have access to the component's variables. Is there some way to inject those variables?

How can I get the read data into the fileString variable in my component?

Answer 1

Do it like this:

myReader.onloadend = (e) => {
   console.log(myReader.result);
   this.fileString = myReader.result as string;
};

So you can access your variables.

For a more detailed explanation: https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Functions/Arrow_functions