The mode read-only property of the Request interface contains the mode of the request (e.g., cors, no-cors, same-origin, navigate or websocket.) This is used to determine if cross-origin requests lead to valid responses, and which properties of the response are readable.

A RequestMode value.

The associated mode, available values of which are:

same-origin

If a request is made to another origin with this mode set, the result is an error. You could use this to ensure that a request is always being made to your origin.

no-cors

Prevents the method from being anything other than HEAD, GET or POST, and the headers from being anything other than CORS-safelisted request headers. If any ServiceWorkers intercept these requests, they may not add or override any headers except for those that are CORS-safelisted request headers. In addition, JavaScript may not access any properties of the resulting Response. This ensures that ServiceWorkers do not affect the semantics of the Web and prevents security and privacy issues arising from leaking data across domains.

cors

Allows cross-origin requests, for example to access various APIs offered by 3rd party vendors. These are expected to adhere to the CORS protocol. Only a limited set of headers are exposed in the Response, but the body is readable.

navigate

A mode for supporting navigation. The navigate value is intended to be used only by HTML navigation. A navigate request is created only while navigating between documents.

websocket

A special mode used only when establishing a WebSocket connection.

Requests can be initiated in a variety of ways, and the mode for a request depends on the particular means by which it was initiated.

For example, when a Request object is created using the Request() constructor, the value of the mode property for that Request is set to cors.

However, for requests created other than by the Request() constructor, no-cors is typically used as the mode; for example, for embedded resources where the request is initiated from markup, unless the crossorigin attribute is present, the request is in most cases made using the no-cors mode — that is, for the <link> or <script> elements (except when used with modules), or <img>, <audio>, <video>, <object>, <embed>, or <iframe> elements.

In the following snippet, we create a new request using the Request() constructor (for an image file in the same directory as the script), then save the request mode in a variable:

const myRequest = new Request("flowers.jpg");
const myMode = myRequest.mode; // returns "cors" by default

• ServiceWorker API
• HTTP access control (CORS)
• HTTP