metamask-extension/app/scripts/lib/createTabIdMiddleware.js

/**
 * Returns a middleware that appends the DApp TabId to the request
 *
 * @param {{ tabId: number }} opts - The middleware options
 * @returns {Function}
 */
export default function createTabIdMiddleware(opts) {
  return function tabIdMiddleware(
    /** @type {any} */ req,
    /** @type {any} */ _,
    /** @type {Function} */ next,
  ) {
    req.tabId = opts.tabId;
    next();
  };
}