Angular 11 is indicating that the type 'File | null' cannot be assigned to the type 'File'

Question

Angular 11 is indicating that the type 'File | null' cannot be assigned to the type 'File'

Hey there, I'm currently diving into Angular and I'm working on an Angular 11 project. My task involves uploading a CSV file, extracting the records on the client side, and saving them in a database through ASP.NET Web API.

I followed a tutorial to retrieve the data from the CSV file on the Angular side and display it on the same page. However, I encountered the following error:

Type 'File | null' is not assignable to type 'File'. Type 'null' is not assignable to type 'File'.ts(2322)

I've tried multiple solutions but haven't been able to resolve it. Any help would be greatly appreciated!

Below is my .ts file:

import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-file-upload',
  templateUrl: './file-upload.component.html',
  styles: [
  ]
})
export class FileUploadComponent implements OnInit {

  constructor() { }

  ngOnInit(): void {
  }

  lines: any = [];
  linesR: any = [];

  changeListener(files: FileList) {
    console.log(files);
    if (files && files.length > 0) {
      let file: File = files.item(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);
      }
    }
  }
}

This is my .html file

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

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

    <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>

Here is the error I encountered

Appreciate any assistance!

angular typescript angular11

Answer 1

Answer №1

Make Use of:

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>

Answer 2

Make Use of:

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>

Answer 3

Answer №2

Here is an alternative approach -

const dataFile: File | any = null;

OR

const dataFile: File | null = null;

Answer 4

Here is an alternative approach -

const dataFile: File | any = null;

OR

const dataFile: File | null = null;

Answer 5

Answer №3

When you are confident that there are values in the files item, you can proceed with the following:

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

Answer 6

When you are confident that there are values in the files item, you can proceed with the following:

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

Answer 7

Answer №4

Utilizing strict: true in TypeScript is a great practice. However, the issue you're encountering is with this specific code snippet:

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

Even though you are checking the length before accessing the item, it's possible for it to return undefined. This causes the type to be File | null, whereas you're specifying it as just File.

A better approach would be to modify it as follows:

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

 if (file) {
   // Within this block, `file` will be recognized as type `File` due to the assertion within the `if` statement
      console.log(file.size);
      console.log(file.type);

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

Answer 8

Utilizing strict: true in TypeScript is a great practice. However, the issue you're encountering is with this specific code snippet:

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

Even though you are checking the length before accessing the item, it's possible for it to return undefined. This causes the type to be File | null, whereas you're specifying it as just File.

A better approach would be to modify it as follows:

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

 if (file) {
   // Within this block, `file` will be recognized as type `File` due to the assertion within the `if` statement
      console.log(file.size);
      console.log(file.type);

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

Answer 9

Answer №5

To set a file to Null, use the following code: file = new File([],''); This will reset the file object to an empty state

Answer 10

To set a file to Null, use the following code: file = new File([],''); This will reset the file object to an empty state

Answer 11

Answer №6

No issues whatsoever with the following code snippet:

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

or with this variation:

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

Furthermore, I have no problem using files[0] in place of files.item(0). All of this is done while adhering to strict mode rules.

Answer 12

No issues whatsoever with the following code snippet:

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

or with this variation:

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

Furthermore, I have no problem using files[0] in place of files.item(0). All of this is done while adhering to strict mode rules.

Angular 11 is indicating that the type 'File | null' cannot be assigned to the type 'File'

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

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