XRInputSourceArray: length property

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.