TypeScript declaration file with the NewRelic Browser typings.

newrelic-browser.d.ts

// Type definitions for NewRelicBrowser v1026
// Project: https://docs.newrelic.com/docs/browser/new-relic-browser/browser-agent-spa-api
// Definitions by: Piotr Kubisa <https://gist.github.com/piotrkubisa/4a1cc7355afcec46651a896d59ec8d50>

declare var newrelic: NewRelicBrowser;

interface NewRelicBrowser {
  addPageAction(name: string, attributes: any): void;
  addRelease(release_name: string, release_id: string): void;
  addToTrace(custom_object: any): void;
  finished(time_stamp: number): void;
  noticeError(error: Error): void;
  setCustomAttribute(name: string, value: string): void;
  setErrorHandler(error: Error, callback: (err: Error) => void): any;
  setPageViewName(name: string, host?: string): void;
  interaction(): NewRelicBrowserIntegration;
  setCurrentRouteName(name: string): void;
}

interface NewRelicBrowserIntegration {
  appId: any;
  appName: any;
  ajaxCount: any;
  asn: any;
  asnLatitude: any;
  asnLongitude: any;
  asnOrganization: any;
  backendTransactionName: any;
  browserInteractionName: any;
  category: any;
  city: any;
  countryCode: any;
  deviceType: any;
  duration: any;
  jsDuration: any;
  previousGroupedUrl: any;
  previousRouteName: any;
  previousURL: any;
  regionCode: any;
  session: any;
  targetGroupedUrl: any;
  targetRouteName: any;
  targetUrl: any;
  timeToConnectStart: any;
  timeToConnectEnd: any;
  timeToDomComplete: any;
  timeToDomContentLoadedEventStart: any;
  timeToDomContentLoadedEventEnd: any;
  timeToDomInteractive: any;
  timeToDomLoading: any;
  timeToDomainLookupStart: any;
  timeToDomainLookupEnd: any;
  timeToFetchStart: any;
  timeToLoadEventStart: any;
  timeToLoadEventEnd: any;
  timeToRedirectStart: any;
  timeToRedirectEnd: any;
  timeToRequestStart: any;
  timeToResponseStart: any;
  timeToResponseEnd: any;
  timeToSecureConnectionStart: any;
  timeToUnloadEventStart: any;
  timeToUnloadEventEnd: any;
  timestamp: any;
  trigger: any;
  userAgentName: any;
  userAgentVersion: any;
  userAgentOS: any;

  createTracer(name: string, callback: () => void): NewRelicBrowserIntegration;
  end(): NewRelicBrowserIntegration;
  getContext(callback: (ctx: NewRelicBrowserIntegration) => void): NewRelicBrowserIntegration;
  ignore(): NewRelicBrowserIntegration;
  onEnd(callback: (ctx: NewRelicBrowserIntegration) => void): NewRelicBrowserIntegration;
  save(): NewRelicBrowserIntegration;
  setAttribute(key: string, value: any): NewRelicBrowserIntegration;
  setName(name: string, trigger?: string): NewRelicBrowserIntegration;
}

Thanks for these definitions, @piotrkubisa. I've extended your typings and published them at https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/new-relic-browser. I didn't check beforehand, but I've included you as one of the authors. Hope that's OK?
The typings can be installed with npm install --save @types/new-relic-browser.