To upload a file without navigating away from the page, submit a form’s FormData in a POST request. Use Fetch for a simple request-and-response flow; use XMLHttpRequest (XHR) if you need upload-progress events. In either case, leave the multipart Content-Type header unset so the browser can add the required boundary. The server must parse the multipart request and validate and safely store the uploaded content.
Build the upload form
Give the file input a name; that name becomes the multipart field the server reads. Setting action, method and enctype also leaves the form configured for a normal browser submission if JavaScript is unavailable.
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<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>Replace /upload with your application’s upload endpoint. A selected file is available from the input as a File object; a filename string by itself is not the file’s contents.
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Construct FormData from the form in its submit handler, prevent the default navigation, and pass the data as the request body. The browser encodes it as multipart form data.
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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);
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);
}
});The success message means the endpoint returned an HTTP success status; the server should return that status only after its own validation and handling have succeeded. If the endpoint returns useful response data, read it from response and display an appropriate result.
Do not set Content-Type yourself. In particular, do not set it to multipart/form-data manually: the browser must include the multipart boundary parameter. MDN Web Docs gives this guidance for FormData requests made with Fetch or XMLHttpRequest: Using FormData Objects.
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Show upload progress with XMLHttpRequest
Fetch accepts FormData, but for transfer-progress notifications use the XHR upload event target. This example reports a percentage when the browser can calculate one, and treats transfer completion separately from the server’s response.
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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", (progressEvent) => {
if (progressEvent.lengthComputable) {
const percent = Math.round((progressEvent.loaded / progressEvent.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 while uploading.";
});
request.addEventListener("abort", () => {
result.textContent = "Upload canceled.";
});
request.open("POST", form.action);
request.send(body);
});Progress reaching 100% only indicates that the browser has finished sending the request body; it does not establish that the server accepted, validated or stored the file. Wait for the request’s response before showing success. MDN notes browser bugs concerning whether upload event listeners need to be attached before or after open(); if precise event timing matters, test the browsers you support. See MDN’s XMLHttpRequest upload property reference.
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- Security & Warranty: Securely protects files using an advanced security software solution with 256-bit AES encryption. Backed by a Lexar 3-year limited warranty
Choose Fetch or XMLHttpRequest
| Need | Use | Reason |
|---|---|---|
| Send FormData and handle the response | Fetch | It accepts FormData as a request body and suits a straightforward asynchronous request/response flow. |
| Display transfer progress | XMLHttpRequest | Its upload object exposes progress events. |
| Support a particular set of older browsers | Check compatibility for those browsers | Browser-version support is not established here; verify it against your actual targets. |
Fetch is not limited to non-file requests: it can send FormData containing files. The practical distinction here is progress reporting.
Handle multiple files and other fields
To let a user select more than one file, add multiple to the file input. Keep a field name your endpoint expects, and ensure the endpoint is implemented to accept and process multiple file parts.
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<input id="upload-files" name="files" type="file" multiple>Creating new FormData(form) includes the selected files and other successful form controls, such as named text inputs. The browser APIs represent form entries and file parts in the multipart request; the server must parse those parts using its own framework or library. The WHATWG Fetch Standard describes the FormData and multipart model.
Validate and store uploads on the server
The browser-side code transports data; it does not make an upload safe. Treat both file contents and submitted filenames as untrusted. Apply application-appropriate size and type policies and validate on the server, even if the page also checks files for user convenience.
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- Use a dedicated upload location, preferably outside the application directory tree, and disable execute permissions there.
- Do not use a client-supplied filename as a storage path. Submitted names may contain full or relative paths; generate a safe storage name instead.
- If you display or log an original filename, encode it appropriately for that context.
- Make sure the backend’s multipart parser and request-size limits are configured for the files your application intends to accept.
Server APIs vary by framework. For example, Microsoft’s ASP.NET Core 10.0 file upload guide shows multipart forms and access to uploaded files through IFormFile; that type is specific to ASP.NET Core, not a universal upload API. Apply the equivalent parsing and safe-storage approach in your own backend.
Troubleshooting common upload failures
- The server says no file was received: Confirm the input has a
name, a file is selected, the request is sent to the intended endpoint, and that endpoint parses multipart form data. Check that its expected field name matches the input’s name. - The server cannot parse the multipart body: If you set a
Content-Typeheader manually, remove it and let the browser generate the header and boundary for the FormData body. - The request returns an error status: Check the endpoint’s response and server logs for its validation, authentication, size-limit or parsing error. The example treats non-2xx responses as failures; a completed transfer is not enough to show success.
- No progress percentage appears: The percentage is only calculated when the XHR progress event has
lengthComputableset to true. Display an indeterminate “Uploading” state otherwise. - Progress completes but the upload fails: Progress reports bytes sent, not server-side acceptance. Handle the response, network error and abort outcomes separately.
- The page navigates instead of using Ajax: Check that the submit listener is attached to the correct form and calls
event.preventDefault(). Also verify that JavaScript has loaded without an earlier error.
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