Baseline Widely available
Note: This feature is only available in Service Workers.
The postMessage() method of the Client interface allows a service worker to send a message to a client (a Window, Worker, or SharedWorker). The message is received in the message event on navigator.serviceWorker.
postMessage(message)
postMessage(message, transfer)
postMessage(message, options)
message
The message to send to the client. This can be any structured-cloneable type.
transfer Optional
An optional array of transferable objects to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.
options Optional
An optional object containing the following properties:
transfer Optional
Has the same meaning as the transfer parameter.
None (undefined).
The code below sends a message from a service worker to a client. The client is fetched using the get() method on clients, which is a global in service worker scope.
addEventListener("fetch", (event) => {
event.waitUntil(
(async () => {
// Exit early if we don't have access to the client.
// Eg, if it's cross-origin.
if (!event.clientId) return;
// Get the client.
const client = await self.clients.get(event.clientId);
// Exit early if we don't get the client.
// Eg, if it closed.
if (!client) return;
// Send a message to the client.
client.postMessage({
msg: "Hey I just got a fetch from you!",
url: event.request.url,
});
})(),
);
});
Receiving that message:
navigator.serviceWorker.addEventListener("message", (event) => {
console.log(event.data.msg, event.data.url);
});
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