When uploading files in web development, there are generally two common methods: using FormData and directly through the request body. Each method has its own characteristics and use cases.

Uploading via FormData

Method: FormData is a way to encode data as a set of key-value pairs. When you use FormData to upload files, it is typically used in combination with multipart/form-data encoding type. This is the standard way to upload files in HTML forms.

// HTML form with input type 'file'
// <input type="file" id="fileInput">

const fileInput = document.getElementById('fileInput');

fileInput.addEventListener('change', function(event) {
    const file = event.target.files[0];
    const formData = new FormData();
    formData.append('file', file);

    fetch('/upload', {
        method: 'POST',
        body: formData
        // Note: no need to explicitly set the Content-Type header here
    })
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error(error));
});

Use Case: It is ideal for uploading files along with other form fields, such as text input, checkboxes, etc.

Compatibility: Widely supported by most browsers and can be used with XMLHttpRequest (XHR) and the newer Fetch API.

Advantages:

• Can handle multiple types of data (files, text, binary data).
• Easy to append additional fields and compatible with traditional form-based web applications.
• Generally, no need to manually set headers as they are automatically set, which includes setting the Content-Type to multipart/form-data and the correct boundary.

Disadvantages:

• Slightly more overhead due to the multipart encoding.

Uploading via Request Body

Method: Directly sending the file data in the body of an HTTP request. Here, you can set the Content-Type header to the file's MIME type (like image/jpeg, application/pdf, etc.) and place the file's raw data in the request body.

// HTML form with input type 'file'
// <input type="file" id="fileInput">

const fileInput = document.getElementById('fileInput');

fileInput.addEventListener('change', function(event) {
    const file = event.target.files[0];

    fetch('/upload', {
        method: 'POST',
        body: file,
        headers: {
            'Content-Type': file.type
        }
    })
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error(error));
});

Use Case: Suited for APIs or services where you're uploading only a file, without any additional form fields.

Compatibility: Works with both XMLHttpRequest and Fetch API. Requires manual setting of headers and handling of file data.

Advantages:

• More efficient for sending only file data as it avoids the overhead of multipart encoding.
• Offers more control over the request, such as setting custom headers.

Disadvantages:

• Requires manual handling of content types and boundaries if needed.

• Not suitable for situations where you need to send additional data alongside the file.

Summary

FormData is best for situations where a file is being uploaded as part of a form with other fields. It's more browser-friendly and straightforward to use. Direct Body Upload is more efficient for APIs where you're only uploading a file and need more control over the request. It's less suited for traditional form-based uploads and requires more manual setup.