Uploading Files via JS Fetch API Complete Guide

A complete guide to uploading files via the JavaScript Fetch API. Covers FormData construction, single and multiple file uploads, drag-and-drop integration, upload progress tracking, chunked uploads for large files, validation before upload, and server-side handling expectations.

JavaScriptintermediate

RuneHub Team

March 3, 2026

12 min read

RuneHub Team

Mar 3, 2026

12 min read

File uploading with the Fetch API uses FormData to construct multipart requests. The browser handles encoding, boundary generation, and streaming automatically. This guide covers every file upload pattern from simple single-file uploads to chunked large-file transfers.

Basic Single File Upload

javascript

const fileInput = document.getElementById("file-input");
 
async function uploadFile(file) {
  const formData = new FormData();
  formData.append("file", file);
 
  const response = await fetch("/api/upload", {
    method: "POST",
    body: formData,
    // Do NOT set Content-Type — browser sets it with boundary
  });
 
  if (!response.ok) {
    throw new Error(`Upload failed: ${response.status}`);
  }
 
  return response.json();
}
 
fileInput.addEventListener("change", async (event) => {
  const file = event.target.files[0];
  if (file) {
    const result = await uploadFile(file);
    console.log("Uploaded:", result.url);
  }
});

The critical rule: never set the Content-Type header manually when using FormData. The browser must generate it with the multipart boundary.

Multiple File Upload

javascript

async function uploadMultiple(files) {
  const formData = new FormData();
 
  // Append each file with the same field name
  for (const file of files) {
    formData.append("files", file);
  }
 
  // Add metadata alongside files
  formData.append("folder", "documents");
  formData.append("tags", JSON.stringify(["report", "2026"]));
 
  const response = await fetch("/api/upload/batch", {
    method: "POST",
    body: formData,
  });
 
  if (!response.ok) {
    throw new Error(`Batch upload failed: ${response.status}`);
  }
 
  return response.json();
}
 
// From a multi-select input
const input = document.getElementById("multi-file");
input.addEventListener("change", (e) => {
  uploadMultiple(e.target.files);
});

Client-Side Validation

Validate files before uploading to save bandwidth and provide instant feedback:

javascript

function validateFile(file, rules = {}) {
  const errors = [];
  const {
    maxSize = 10 * 1024 * 1024, // 10 MB default
    allowedTypes = [],
    allowedExtensions = [],
  } = rules;
 
  // Size check
  if (file.size > maxSize) {
    const maxMB = (maxSize / (1024 * 1024)).toFixed(1);
    errors.push(`File size ${(file.size / (1024 * 1024)).toFixed(1)} MB exceeds limit of ${maxMB} MB`);
  }
 
  // MIME type check
  if (allowedTypes.length > 0 && !allowedTypes.includes(file.type)) {
    errors.push(`File type "${file.type}" is not allowed. Accepted: ${allowedTypes.join(", ")}`);
  }
 
  // Extension check
  if (allowedExtensions.length > 0) {
    const ext = file.name.split(".").pop().toLowerCase();
    if (!allowedExtensions.includes(ext)) {
      errors.push(`Extension ".${ext}" is not allowed. Accepted: ${allowedExtensions.join(", ")}`);
    }
  }
 
  return { valid: errors.length === 0, errors };
}
 
// Usage
const validation = validateFile(file, {
  maxSize: 5 * 1024 * 1024,
  allowedTypes: ["image/jpeg", "image/png", "image/webp"],
  allowedExtensions: ["jpg", "jpeg", "png", "webp"],
});
 
if (!validation.valid) {
  console.error(validation.errors);
  return;
}

Validation	Client-Side	Server-Side
File size	Fast feedback	Enforce hard limit
MIME type	Quick filter	Verify with magic bytes
Extension	UI-level filter	Whitelist enforcement
Virus scan	Not possible	Required for security

Always validate on both client and server. Client-side validation is a UX convenience, not a security measure.

Drag-and-Drop Upload

javascript

const dropZone = document.getElementById("drop-zone");
 
dropZone.addEventListener("dragover", (e) => {
  e.preventDefault();
  dropZone.classList.add("drag-active");
});
 
dropZone.addEventListener("dragleave", () => {
  dropZone.classList.remove("drag-active");
});
 
dropZone.addEventListener("drop", async (e) => {
  e.preventDefault();
  dropZone.classList.remove("drag-active");
 
  const files = Array.from(e.dataTransfer.files);
  if (files.length === 0) return;
 
  for (const file of files) {
    const validation = validateFile(file, { maxSize: 10 * 1024 * 1024 });
    if (!validation.valid) {
      showError(file.name, validation.errors);
      continue;
    }
    await uploadFile(file);
  }
});

Image Preview Before Upload

javascript

function previewImage(file) {
  return new Promise((resolve) => {
    const reader = new FileReader();
    reader.onload = (e) => resolve(e.target.result);
    reader.readAsDataURL(file);
  });
}
 
const imageInput = document.getElementById("image-input");
const preview = document.getElementById("preview");
 
imageInput.addEventListener("change", async (e) => {
  const file = e.target.files[0];
  if (file && file.type.startsWith("image/")) {
    const dataUrl = await previewImage(file);
    preview.src = dataUrl;
    preview.style.display = "block";
  }
});

Upload Progress Tracking

Fetch does not natively support upload progress. Use XMLHttpRequest for progress events:

javascript

function uploadWithProgress(url, formData, onProgress) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest();
 
    xhr.upload.addEventListener("progress", (event) => {
      if (event.lengthComputable) {
        const percent = Math.round((event.loaded / event.total) * 100);
        onProgress(percent, event.loaded, event.total);
      }
    });
 
    xhr.addEventListener("load", () => {
      if (xhr.status >= 200 && xhr.status < 300) {
        resolve(JSON.parse(xhr.responseText));
      } else {
        reject(new Error(`Upload failed: ${xhr.status}`));
      }
    });
 
    xhr.addEventListener("error", () => reject(new Error("Network error")));
    xhr.addEventListener("abort", () => reject(new Error("Upload cancelled")));
 
    xhr.open("POST", url);
    xhr.send(formData);
  });
}
 
// Usage
const formData = new FormData();
formData.append("file", file);
 
await uploadWithProgress("/api/upload", formData, (percent, loaded, total) => {
  progressBar.style.width = `${percent}%`;
  progressText.textContent = `${percent}% (${(loaded / 1024 / 1024).toFixed(1)} MB / ${(total / 1024 / 1024).toFixed(1)} MB)`;
});

Chunked Upload for Large Files

Split large files into chunks for reliable uploads with resume capability:

javascript

async function chunkedUpload(file, chunkSize = 5 * 1024 * 1024) {
  const totalChunks = Math.ceil(file.size / chunkSize);
  const uploadId = crypto.randomUUID();
 
  for (let i = 0; i < totalChunks; i++) {
    const start = i * chunkSize;
    const end = Math.min(start + chunkSize, file.size);
    const chunk = file.slice(start, end);
 
    const formData = new FormData();
    formData.append("chunk", chunk);
    formData.append("uploadId", uploadId);
    formData.append("chunkIndex", i.toString());
    formData.append("totalChunks", totalChunks.toString());
    formData.append("fileName", file.name);
 
    const response = await fetch("/api/upload/chunk", {
      method: "POST",
      body: formData,
    });
 
    if (!response.ok) {
      throw new Error(`Chunk ${i} failed: ${response.status}`);
    }
 
    console.log(`Chunk ${i + 1}/${totalChunks} uploaded`);
  }
 
  // Finalize — tell server to assemble chunks
  const response = await fetch("/api/upload/finalize", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ uploadId, fileName: file.name, totalChunks }),
  });
 
  return response.json();
}

Rune AI

Key Insights

Never set Content-Type for FormData: The browser must generate the multipart boundary string automatically
Validate client-side first, server-side always: Client validation is UX; server validation is security
Fetch has no upload progress API: Use XMLHttpRequest's upload.onprogress event for progress bars
Chunked uploads enable large files and resume: Split files into 5-10 MB chunks with a server-side assembly step
Drag-and-drop uses DataTransfer.files: Access dropped files from the event's dataTransfer property after preventing default behavior

Frequently Asked Questions

Why does setting Content-Type manually break FormData uploads?

The browser generates a unique boundary string for multipart encoding. When you set Content-Type manually, you override this boundary, and the server cannot parse the multipart body.

How do I upload a Blob or canvas image?

Convert to a File-like Blob: `formData.append("image", blob, "screenshot.png")`. The third argument sets the filename.

Can I cancel an ongoing upload?

With Fetch, use `AbortController`. With XHR, call `xhr.abort()`. Both immediately terminate the upload and close the connection.

What is the maximum file size for fetch uploads?

There is no hard limit in Fetch itself. The limit is set by the server configuration (e.g., nginx `client_max_body_size`) and available memory. For very large files, use chunked uploads.

Conclusion

File uploads with Fetch use FormData for automatic multipart encoding. Always validate files client-side before uploading, never set Content-Type manually with FormData, and use chunked uploads for large files. For upload progress, fall back to XHR. For the full Fetch API, see how to use the JS fetch API complete tutorial. For POST request patterns beyond file uploads, see handling POST requests with JS fetch API guide.

Uploading Files via JS Fetch API Complete Guide

Basic Single File Upload

Multiple File Upload

Client-Side Validation

Drag-and-Drop Upload

Image Preview Before Upload

Upload Progress Tracking

Chunked Upload for Large Files

Rune AI

Frequently Asked Questions

Why does setting Content-Type manually break FormData uploads?

How do I upload a Blob or canvas image?

Can I cancel an ongoing upload?

What is the maximum file size for fetch uploads?

Conclusion

Tags

More in this topic

OffscreenCanvas API in JS for UI Performance

Advanced Web Workers for High Performance JS

JavaScript Macros and Abstract Code Generation

Uploading Files via JS Fetch API Complete Guide

Basic Single File Upload

Multiple File Upload

Client-Side Validation

Drag-and-Drop Upload

Image Preview Before Upload

Upload Progress Tracking

Chunked Upload for Large Files

Rune AI

Frequently Asked Questions

Why does setting Content-Type manually break FormData uploads?

How do I upload a Blob or canvas image?

Can I cancel an ongoing upload?

What is the maximum file size for fetch uploads?

Conclusion

Tags

More in this topic

OffscreenCanvas API in JS for UI Performance

Advanced Web Workers for High Performance JS

JavaScript Macros and Abstract Code Generation