In this quick tutorial, you'll learn how to work with FormData in Angular 7 and how to post it to a web server via a POST request and HttpClient.

One of the most important aspects of web development is forms as they allow you to collect data from users and send it to servers.

There are various ways to work with forms in JavaScript and HTML. Also different frameworks (such as Angular) added other ways to handle forms.

In this tutorial, we'll be looking at FormData, A browser API for handling forms data just like its name suggests. This API provides methods and properties that enable to have access and work with form elements and their values in a straightforward way.

It's particularly helpful if you are working with client side frameworks like Angular as it allows you to easily prepare form data to be sent with POST HTTP requests.

Note: You can think of FormData as a reprsentation of an HTML form in JavaScript instead of HTML.

You also can create a FormData instance from an HTML form

The FormData API allows you to create a set of key/value elements that correspond to form fields and their values. This can be then sent to the server using Angular HttpClient.

Note: A FormData instance is equivalent to an HTML form sent using the multipart/form-data encoding.

How to use FormData in Angular 7

Let's now see an example of how you can create a FormData instance and send it with HttpClient POST in Angular 7.

Note: We assume that you have a server running at the http://localhost:3000 address with an /upload that accepts POST requests for uploading files in your server.

If you would like to create a server for uploading files, check out Nest.js Tutorial: File Uploading with Multer and Serving Static Files in Nest.

Provided that you have created an Angular 7 project with Angular CLI. Navigate to your project's root folder and run the following command to generate a component that we'll be working with:

$ ng generate component upload

Open the src/app/upload/upload.component.html file and add the following form:

<h1>Angular 7 FormData (multipart/data-form) Example</h1>
<div>
    <form [formGroup] = "uploadForm" (ngSubmit)="onSubmit()">      
      <div>
        <input type="file" name="profile" (change)="onFileSelect($event)" />
      </div>
      <div>
        <button type="submit">Upload</button>
      </div>
    </form>
  </div>
</div>

Next, open the src/app/upload/upload.component.ts file and start by importing the necessary APIs:

import { FormBuilder, FormGroup } from '@angular/forms';
import { HttpClient } from '@angular/common/http';

We import FormBuilder and FormGroup from the @angular/forms package which are necessary to create a reactive form in Angular. We also import HttpClient that will be used to send data to the server.

Note: Make sure to import ReactiveFormsModule and HttpClientModule in your main module application which exists in the src/app/app.module.ts file and add them to the imports array.

Using HttpClient calls directly from your components is against the separation of concerns principle but this is just a simple example. Typically, you would need to create a service and make HttpClient from the service.

Next define the SERVER_URL and uploadForm variables in your component:

export class UploadComponent implements OnInit {

  SERVER_URL = "http://localhost:3000/upload";
  uploadForm: FormGroup;  

Next, import and inject HttpClient and FormBuilder:

export class UploadComponent implements OnInit {
  constructor(private formBuilder: FormBuilder, private httpClient: HttpClient) { }

Next, create a reactive form in ngOnInit() method of the component which gets called when the component is initialized:

  ngOnInit() {
    this.uploadForm = this.formBuilder.group({
      profile: ['']
    });
  }

Next, let's add the onFileSelect() method which gets called when a file is selected by the user:

  onFileSelect(event) {
    if (event.target.files.length > 0) {
      const file = event.target.files[0];
      this.uploadForm.get('profile').setValue(file);
    }
  }

We simply check if at least one file is selected and we set the profile field of uploadForm to the selected file.

Finally, let's see how we can use FormData to send multipart/form-data to our server. Let's define the onSubmit() method:

  onSubmit() {
    const formData = new FormData();
    formData.append('file', this.uploadForm.get('profile').value);

    this.httpClient.post<any>(this.SERVER_URL, formData).subscribe(
      (res) => console.log(res),
      (err) => console.log(err)
    );
  }

We simply create an instance of FormData, next we add fields with their values using the append() method of FormData. In our example, we only add a field named file which holds the value the selected file. Finally we use the post() method of HttpClient to send the form data to the server.

For reference, FormData provides the following methods for working with form data:

• The FormData.append() appends a new value for an existing key, or adds the key if it does not exist.
• The FormData.delete() method deletes a key/value pair from a FormData object.
• The FormData.entries()method provides an iterator for going through all key/value pairs of the instance.
• The FormData.get() method Returns the first value associated with a given key from within a FormData object.
• The FormData.getAll() method provides an array of all the values associated with a specific key.
• The FormData.has() methods provides a boolean indicating whether a FormData instance contains a specific key.
• The FormData.keys() method provides an iterator for going through all the keys contained in the form instance.
• The FormData.set()method sets a new value for an existing key inside a FormData object, or adds the key/value if it does not exist.
• The FormData.values() method provides an iterator for going through all values contained in this object.