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

/**
 * Returns a middleware that appends the DApp origin to request
 * @param {{ origin: string }} opts - The middleware options
 * @returns {Function}
 */
export default function createOriginMiddleware(opts) {
  return function originMiddleware(
    /** @type {any} */ req,
    /** @type {any} */ _,
    /** @type {Function} */ next,
  ) {
    req.origin = opts.origin
    next()
  }
}