The read-only length property returns an integer value indicating the number of items in the input source list represented by the XRInputSourceArray object.

An integer value indicating the number of XRInputSource objects representing WebXR input sources are included in the array.

In this example, a game that requires at least one input source uses length to check this before proceeding to allow the user to play the game.

let sources = xrSession.inputSources;

if (sources.length === 0) {
  showAlertDialog(
    "You need to have at least one controller to play Super Duper Shark Jump Fest 9000.",
    [
      { label: "Shop Now", url: "https://www.example.com/shop/controllers" },
      { label: "Quit", handler: quitGame },
    ],
  );
}

Here, if length is 0, a hypothetical showAlertDialog() function is called with a prompt string explaining the need for a controller, and an array of objects, each describing a button and what should happen when it's clicked. The first takes the user to an Amazon.com search for VR controllers, and the second calls a quitGame() function to start shutting the game program down.