File Download

Use this skill when the user asks how to let the browser download a file — whether it lives in the app filesystem, is generated at runtime, or comes from an external URL.

Downloading App Filesystem Files

The server serves authenticated files directly at their layer paths. Use a URL built from location.href so the request carries the session cookie and the server enforces read permissions.

From the authenticated user's home folder (~/)

/~/... maps to L2/<username>/... for the currently logged-in user.

const u = new URL(location.href);
u.pathname = '/~/BTC_ETH_ratio_chart.pdf';
const a = document.createElement('a');
a.href = u.toString();
a.download = 'BTC_ETH_ratio_chart.pdf';
a.click();

From a specific layer path (/L0/, /L1/, /L2/)

Use the full layer path directly. The server checks that the authenticated user has read access before serving.

function downloadAppFile(layerPath, filename) {
  // layerPath example: 'L0/_all/mod/_core/reports/template.pdf'
  const u = new URL(location.href);
  u.pathname = `/${layerPath}`;
  const a = document.createElement('a');
  a.href = u.toString();
  a.download = filename;
  a.click();
}

// Examples:
downloadAppFile('L0/_all/mod/_core/reports/template.pdf', 'template.pdf');
downloadAppFile('L2/alice/exports/summary.csv', 'summary.csv');

Read permissions follow the same rules as the file APIs:

• L2/<username>/ — own files only
• L0/<group>/ and L1/<group>/ — group members only
• Unauthenticated requests return 401; unauthorized paths return 403

Downloading Runtime-Generated In-Memory Content

For files generated on the fly (CSV exports, JSON dumps, dynamically built text), create a Blob, make an object URL, and revoke it after the click.

function downloadBlob(content, filename, mimeType = 'text/plain') {
  const blob = new Blob([content], { type: mimeType });
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
  URL.revokeObjectURL(url);
}

// Plain text
downloadBlob('hello world', 'note.txt', 'text/plain');

// CSV
const csv = 'name,value\nalice,42\nbob,17';
downloadBlob(csv, 'data.csv', 'text/csv');

// JSON
const json = JSON.stringify({ status: 'ok', items: [1, 2, 3] }, null, 2);
downloadBlob(json, 'result.json', 'application/json');

// Binary data from a Uint8Array or ArrayBuffer
const bytes = new Uint8Array([0x25, 0x50, 0x44, 0x46]); // %PDF header
downloadBlob(bytes, 'output.pdf', 'application/pdf');

Downloading External Files

Fetch the remote file through the server proxy so the request is not blocked by CORS, then turn the response into a Blob download. This works for any URL the server can reach.

async function downloadExternalFile(externalUrl, filename) {
  const response = await space.api.call('proxy', {
    method: 'POST',
    body: { url: externalUrl }
  });

  if (!response.ok) throw new Error(`Fetch failed: ${response.status}`);

  const blob = await response.blob();
  const url = URL.createObjectURL(blob);
  const a = document.createElement('a');
  a.href = url;
  a.download = filename;
  a.click();
  URL.revokeObjectURL(url);
}

// Example:
await downloadExternalFile('https://example.com/report.pdf', 'report.pdf');

If the external URL is public and CORS allows direct access from the browser, you can skip the proxy and fetch it directly with fetch(externalUrl) and the same Blob pattern above.