Type 'File | null' is not assignable to type 'File'. in angular 11

Try Using:

fileupload.component.ts

import { Component, OnInit, VERSION } from "@angular/core";
    
@Component({
  selector: 'app-file-upload',
  templateUrl: './file-upload.component.html',
  styles: []
})
export class FileUploadComponent implements OnInit {
      name = "Angular " + VERSION.major;
    
      lines: any = [];
      linesR: any = [];
    
      ngOnInit() {}
    
      changeListener(files) {
        let fileList = (<HTMLInputElement>files.target).files;
        if (fileList && fileList.length > 0) {
          let file: File = fileList[0];
          console.log(file.name);
          console.log(file.size);
          console.log(file.type);
    
          let reader: FileReader = new FileReader();
          reader.readAsText(file);
          reader.onload = e => {
            let csv: any = reader.result;
            let allTextLines = [];
            allTextLines = csv.split(/\r|\n|\r/);
    
            let headers = allTextLines[0].split(";");
            let data = headers;
            let tarr = [];
            for (let j = 0; j < headers.length; j++) {
              tarr.push(data[j]);
            }
            this.lines.push(tarr);
            let tarrR = [];
            let arrl = allTextLines.length;
            let rows = [];
            for (let i = 1; i < arrl; i++) {
              rows.push(allTextLines[i].split(";"));
            }
            for (let j = 0; j < arrl; j++) {
              tarrR.push(rows[j]);
            }
            this.linesR.push(tarrR);
          };
        }
      }
    }

fileupload.component.html

<div class="container">
  <h1 style="text-align: center">File Upload</h1>
  <br /><br />

  <input
    class="form-control"
    type="file"
    class="upload"
    (change)="changeListener($event)"
  />

  <table class="table table-bordered table-dark">
    <thead>
      <tr>
        <th *ngFor="let item of lines[0]; let i = index">
          {{item}}
        </th>
      </tr>
    </thead>
    <tbody>
      <tr *ngFor="let item of linesR[0]; let i = index">
        <td *ngFor="let itemm of lines[0]; let j = index">
          {{item[j]}}
        </td>
      </tr>
    </tbody>
  </table>
</div>

If you are sure about that you will have values into files item then you can say :

let file: File = files.item(0) as File;

You are using strict: true with typescript. That's good! The issue you are facing though is that this piece:

let file: File = files.item(0);

can return undefined, even though you check for the length before. This would make the type File | null , but you are stating it should just be File.

Better would be to change it to the following:

changeListener(files: FileList) {
 const file: File | null = files?.item(0);

 if (file) {
   // here `file` will have type `File` because it has been asserted with the `if`
      console.log(file.size);
      console.log(file.type);

      let reader: FileReader = new FileReader();
      reader.readAsText(file);
      reader.onload = (e) => {
  //...
 

I've got no problem with this:

const file = files.item(0);
if (file && file !== null) {
  ...

and this:

const file: any = files.item(0);
if (file && file !== null) {
  ...

And even if I'm using files[0] instead of files.item(0). And all this in strict mode.

Another solution is like -

let file: File | any = null;

OR

let file: File | null = null;

If u want to make a file Null do this file= new File([],''); It will reinitialize the file object to an empty file object