The Tool Desk
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Set up the file form
Give the file input a name; that name identifies the file part sent to the server. Include enctype="multipart/form-data" so the form also works as a regular browser submission if JavaScript is unavailable. The server route must be configured to accept and parse multipart form data.
<form id="upload-form" action="/upload" method="post" enctype="multipart/form-data">
<label for="upload-file">Choose a file</label>
<input id="upload-file" name="file" type="file" required>
<button type="submit">Upload</button>
</form>
<output id="upload-result" aria-live="polite"></output>To let the user select more than one file, add multiple to the file input. The server must also support receiving and handling multiple file parts.
Upload with Fetch
In the submit handler, prevent the default page navigation and pass the form to new FormData(form). The resulting object includes the selected file and other successful form controls, such as named text inputs. Send it as the body of a POST request:
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const form = document.querySelector("#upload-form");
const result = document.querySelector("#upload-result");
form.addEventListener("submit", async (event) => {
event.preventDefault();
const body = new FormData(form);
result.textContent = "Uploading…";
try {
const response = await fetch(form.action, {
method: "POST",
body
});
if (!response.ok) {
throw new Error(`Upload failed: ${response.status}`);
}
result.textContent = "Upload complete.";
} catch (error) {
result.textContent = "Upload failed. Please try again.";
console.error(error);
}
});Do not set the request’s Content-Type header yourself. The browser must add the multipart boundary that separates parts in the request body. MDN’s FormData guidance specifically warns against explicitly setting this header when submitting FormData with Fetch or XMLHttpRequest.
Check response.ok or the response status: Fetch resolves its promise for HTTP error responses such as 400 or 500, so a rejected promise alone does not cover all failures. The example reports success when the server returns a successful HTTP status; adapt it to read a response body if the endpoint returns details or a stored-file URL.
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Show upload progress with XMLHttpRequest
Fetch accepts FormData for uploads, but if the interface needs transfer-progress notifications, use the upload event target on an XMLHttpRequest. A progress event reports transferred bytes; it does not prove that the server has validated or stored the file.
const form = document.querySelector("#upload-form");
const result = document.querySelector("#upload-result");
form.addEventListener("submit", (event) => {
event.preventDefault();
const request = new XMLHttpRequest();
const body = new FormData(form);
request.upload.addEventListener("progress", (event) => {
if (event.lengthComputable) {
const percent = Math.round((event.loaded / event.total) * 100);
result.textContent = `Uploading: ${percent}%`;
} else {
result.textContent = "Uploading…";
}
});
request.addEventListener("load", () => {
if (request.status >= 200 && request.status < 300) {
result.textContent = "Upload complete.";
} else {
result.textContent = `Upload failed: ${request.status}`;
}
});
request.addEventListener("error", () => {
result.textContent = "Network error during upload.";
});
request.addEventListener("abort", () => {
result.textContent = "Upload canceled.";
});
request.open("POST", form.action);
request.send(body);
});Register upload listeners before sending. MDN notes browser bugs concerning whether upload listeners need to be attached before or after open(); if exact event timing matters, test the browsers your application supports. See MDN’s XMLHttpRequest upload property documentation.
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What the server must do
The browser sends file bytes as a multipart part; the server endpoint must parse that request and apply the application’s rules. FormData carries the actual selected File, not merely a filename string. Client-side checks can improve feedback, but they cannot be the security boundary.
- Set size and file-type limits appropriate to the application, then validate uploads on the server.
- Store files in a dedicated location, preferably outside the application directory tree, and disable execute permissions there where possible.
- Do not use the submitted filename as a storage path. Treat filenames and file contents as untrusted; encode a filename appropriately if displaying it or writing it to logs.
- Use the multipart parsing and binding mechanism for your backend. For example, Microsoft’s ASP.NET Core 10.0 guide uses
IFormFile; that framework-specific type is not a general browser or server API.
Microsoft’s ASP.NET Core 10.0 upload guide covers multipart forms, multiple uploads, server handling, and security considerations. The Fetch Standard describes the request and FormData model.
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Choose Fetch or XMLHttpRequest
| Need | Choose | Why |
|---|---|---|
| Send FormData and handle the server response | Fetch | It accepts FormData as a request body and supports a simple promise-based flow. |
| Show transfer progress | XMLHttpRequest | Its upload object exposes progress events. |
| Support a particular older browser set | Verify compatibility for those browsers | Browser-version support is not established here; test the targets you actually support. |
Troubleshooting Ajax file uploads
- The server says no file was received: Confirm the file input has a
name, the user selected a file, and the server parser expects the same multipart field name. Ensure the route accepts multipart form data. - The request fails after manually setting Content-Type: Remove the explicit
Content-Type: multipart/form-dataheader and let the browser generate it with the boundary. - The page reloads instead of uploading asynchronously: Confirm the submit listener is attached to the right form and calls
event.preventDefault(). - The UI reports success for a rejected upload: Check the HTTP status. With Fetch, inspect
response.ok; with XMLHttpRequest, check the status in theloadhandler. - Progress does not appear: Check that the upload listener is attached before
send(), and confirm that progress events are available in the target browsers. A percentage can be calculated only whenlengthComputableis true. - Multiple selection works in the picker but not on the server: Add
multipleto the input and ensure the endpoint and its multipart parser handle multiple file parts. - A valid-looking file is rejected or unsafe to store: Browser-side file checks are only a convenience. Apply the relevant type, size, and storage controls on the server.
Or skip the browser setup
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