JavaScript PushManager supportedContentEncodings Property

What You’ll Learn

PushManager.supportedContentEncodings is a read-only static property on PushManager that returns a frozen array of content codings for encrypting push payloads—usually "aes128gcm". Learn why your app server needs it, how to send it with a PushSubscription, the TypeError when modifying the array, and MDN’s server-post pattern—with five examples and try-it labs.

Introduction

Web push messages are encrypted end-to-end between your application server and the browser. The browser generates keys; only the public key and auth secret are shared with your server via PushSubscription.

The server also needs to know which encryption coding to use. PushManager.supportedContentEncodings answers that question—it lists codings this user agent supports, such as aes128gcm from RFC 8291.

Understanding supportedContentEncodings

A read-only static data property on PushManager returning supported push payload encodings.

• Type — array of strings.
• Static — PushManager.supportedContentEncodings, not on instances.
• Frozen — MDN: the array may not be modified; writes throw TypeError.
• Required coding — user agents must support aes128gcm.
• Server use — pick a supported coding when encrypting; set Content-Encoding on push HTTP requests.
• Secure context — Push API features require HTTPS (or localhost).
• Workers — available in Web Workers per MDN.
• Baseline 2025 — newly available since January 2025.

📝 Syntax

PushManager.supportedContentEncodings

Return value

An array of strings. MDN: this usually contains just one value: "aes128gcm".

Exceptions

TypeError — thrown when attempting to set a value in the returned (frozen) array.

⚖️ supportedContentEncodings vs Related APIs

Examples Gallery

Examples follow MDN supportedContentEncodings. Use View Output or Try It Yourself.

if (typeof PushManager !== "undefined") {
  console.log(PushManager.supportedContentEncodings);
  // Usually: ["aes128gcm"]
} else {
  console.log("PushManager not available");
}

["aes128gcm"]

const encodings = PushManager.supportedContentEncodings;
const supportsAes = encodings.includes("aes128gcm");

console.log(supportsAes); // true

true

const encodings = PushManager.supportedContentEncodings;

try {
  encodings[0] = "custom";
} catch (err) {
  console.log(err.name); // "TypeError"
}

TypeError

const pushSubscription =
  await serviceWorkerRegistration.pushManager.subscribe();

const subscriptionObject = {
  endpoint: pushSubscription.endpoint,
  keys: {
    p256dh: pushSubscription.getKey("p256dh"),
    auth: pushSubscription.getKey("auth"),
  },
  encoding: PushManager.supportedContentEncodings,
};

fetch("https://example.com/push/", {
  method: "POST",
  body: JSON.stringify(subscriptionObject),
});

{
  "endpoint": "https://…",
  "keys": { "p256dh": "…", "auth": "…" },
  "encoding": ["aes128gcm"]
}

function getSupportedEncodings() {
  if (!window.isSecureContext) {
    return null;
  }
  if (typeof PushManager === "undefined") {
    return null;
  }
  if (!PushManager.supportedContentEncodings) {
    return null;
  }
  return PushManager.supportedContentEncodings;
}

console.log(getSupportedEncodings());

["aes128gcm"]

🚀 Common Use Cases

• Tell your application server which Content-Encoding to use when encrypting pushes.
• Include encodings in the JSON body posted after pushManager.subscribe().
• Validate server-side encryption libraries against client-supported codings.
• Document push setup for beginners learning the full subscribe → encrypt → deliver flow.
• Pair with getKey() and endpoint for complete server configuration.

🔧 How It Works

📝 Notes

• Baseline 2025 (MDN: newly available since January 2025).
• Not Deprecated, Experimental, or Non-standard — no status banner required.
• Static property — use PushManager.supportedContentEncodings only.
• Frozen array — do not push or assign; copy if you need a mutable list.
• Secure context — HTTPS or localhost for Push API.
• Related learning: PushEvent.data, PushEvent(), navigator.serviceWorker, JavaScript hub.

💡 Best Practices