began adding capability flags

app/capabilities.js

@@ -0,0 +1,74 @@
+async function checkCrypto() {
+  try {
+    const key = await crypto.subtle.generateKey(
+      {
+        name: 'AES-GCM',
+        length: 128
+      },
+      true,
+      ['encrypt', 'decrypt']
+    );
+    await crypto.subtle.encrypt(
+      {
+        name: 'AES-GCM',
+        iv: crypto.getRandomValues(new Uint8Array(12)),
+        tagLength: 128
+      },
+      key,
+      new ArrayBuffer(8)
+    );
+    await crypto.subtle.importKey(
+      'raw',
+      crypto.getRandomValues(new Uint8Array(16)),
+      'PBKDF2',
+      false,
+      ['deriveKey']
+    );
+    await crypto.subtle.importKey(
+      'raw',
+      crypto.getRandomValues(new Uint8Array(16)),
+      'HKDF',
+      false,
+      ['deriveKey']
+    );
+    return true;
+  } catch (err) {
+    return false;
+  }
+}
+
+function checkStreams() {
+  try {
+    new ReadableStream({
+      pull() {}
+    });
+    return true;
+  } catch (e) {
+    return false;
+  }
+}
+
+function polyfillStreams() {
+  try {
+    require('@mattiasbuelens/web-streams-polyfill');
+    return true;
+  } catch (e) {
+    return false;
+  }
+}
+
+export default async function capabilities() {
+  const crypto = await checkCrypto();
+  const nativeStreams = checkStreams();
+  const polyStreams = nativeStreams ? false : polyfillStreams();
+
+  return {
+    crypto,
+    streamUpload: nativeStreams || polyStreams,
+    streamDownload:
+      nativeStreams &&
+      'serviceWorker' in navigator &&
+      !/safari/i.test(navigator.userAgent),
+    multifile: nativeStreams || polyStreams
+  };
+}