`ttvc`

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Overview
Get started
Usage
API
- Types
- Functions
Browser Support
How does it work?
Developing

Overview

ttvc provides an in-browser implementation of the VisuallyComplete metric suitable for field data collection (real-user monitoring).

Visually Complete measures the moment in time when users perceive that all the visual elements of a page have completely loaded. Once the page has reached visually complete, nothing else should change in the viewport without the user’s input.

Get started

This library is available from npm. Add it to your project using the npm or yarn package managers.

$ npm install @dropbox/ttvc

$ yarn add @dropbox/ttvc

Usage

Basic usage

import {init, onTTVC} from '@dropbox/ttvc';

// Call this as early in pageload as possible to setup instrumentation.
init({
  debug: false,
  idleTimeout: 2000,
  networkTimeout: 0,
});

// Reports the last visible change for each navigation that
// occurs during the life of this document.
const unsubscribe = onTTVC((measurement) => {
  console.log('TTVC:', measurement.duration);
});

Report metrics to a collection endpoint

import {init, onTTVC} from '@dropbox/ttvc';

init();

let measurements = [];

// capture measurements in client
onTTVC((measurement) => {
  measurements.append(measurement);
});

// flush data to server when page is hidden
document.addEventListener('visibilitychange', () => {
  if (document.visibilityState === 'hidden') {
    navigator.sendBeacon('/log', JSON.stringify(measurements));
    measurements = [];
  }
});

Record a PerformanceTimeline entry

Capture a span using the Performance Timeline API.

NOTE: Setting arbitrary start and end times with performance.measure relies on the User Timing Level 3 specification. This is not yet adopted by all major browsers.

import {init, onTTVC} from '@dropbox/ttvc';

init();

onTTVC(({start, end, duration, detail}: Metric) => {
  window.performance.measure('TTVC', {
    start,
    end,
    duration,
    detail,
  });
});

Client-side navigation with React Router

@dropbox/ttvc supports measuring client-side navigations!

What counts as navigation may be different in each application, but as long as you signal that a navigation has begun, this library can figure out the rest.

To trigger a new navigation measurement, call start() or dispatch a "locationchange" event on the window object.

// analytics.js
import {init, onTTVC} from '@dropbox/ttvc';

init();

onTTVC((measurement) => {
  console.log('TTVC:', measurement.duration);
});

// app.js
import {start} from '@dropbox/ttvc';
import React, {useEffect} from 'react';
import ReactDOM from 'react-dom';
import {BrowserRouter, useLocation} from 'react-router-dom';

ReactDOM.render(
  <BrowserRouter>
    <App />
  </BrowserRouter>,
  document.getElementById('root')
);

const App = () => {
  const location = useLocation();

  useEffect(() => {
    // Option 1: If you have access to the ttvc library, import it and
    // call start().
    start();

    // Option 2: Dispatch a custom 'locationchange' event. TTVC subscribes to
    // this and will call start() for you.
    window.dispatchEvent(new Event('locationchange'));
  }, [location]);

  return (
    <div className="App">
      <h1>Welcome to React Router!</h1>
      <Routes>
        <Route path="/" element={<Home />} />
        {/* ... more routes */}
      </Routes>
    </div>
  );
};

Attributing TTVC measurement cancellations

In certain cases, @dropbox/ttvc might discard the measurement before it is captured.

This can happen if a user interacts with or navigates away from the page, or the page is put in the background before it has reached a state ot visual completeness.

This is done to obtain a higher confidence of the measurement's accuracy, as interaction with a page can cause it to change in ways that invalidate the measurement.

However, @dropbox/ttvc provides a way to monitor these cancellations and attribute them to a specific cause. A second callback function provided to onTTVC will be called when the measurement is cancelled.

import {init, onTTVC} from '@dropbox/ttvc';

init();

onTTVC(
  (measurement) => {
    console.log('TTVC measurement captured:', measurement.duration);
  },
  (error) => {
    console.log('TTVC measurement cancelled:', error.cancellationReason);
  }
);

API

Types

`Metric`

export type Metric = {
  // time since timeOrigin that the navigation was triggered
  // (this will be 0 for the initial pageload)
  start: number;

  // time since timeOrigin that ttvc was marked for the current navigation
  end: number;

  // the difference between start and end; this is the value of "TTVC"
  duration: number;

  // additional metadata related to the current navigation
  detail: {
    // if ttvc ignored a stalled network request, this value will be true
    didNetworkTimeOut: boolean;

    // the most recent visible update
    // (this can be either a mutation or a load event target, whichever
    // occurred last)
    lastVisibleChange?: HTMLElement | TimestampedMutationRecord;

    // describes how the navigation being measured was initiated
    // NOTE: this extends the navigation type values defined in the W3 spec;
    // "script" is usually reported as "navigation" by the browser, but we
    // report that distinctly
    // @see https://developer.mozilla.org/en-US/docs/Web/API/PerformanceNavigationTiming/type
    navigationType: // Navigation started by clicking a link, by entering the
    // URL in the browser's address bar, or by form submission.
    | 'navigate'
      // Navigation is through the browser's reload operation.
      | 'reload'
      // Navigation is through the browser's history traversal operation.
      | 'back_forward'
      // Navigation is initiated by a prerender hint.
      | 'prerender'
      // Navigation is triggered with a script operation, e.g. in a single page application.
      | 'script';
  };
};

`CancellationError`

export type CancellationError = {
  // time since timeOrigin that the navigation was triggered
  start: number;

  // time since timeOrigin that cancellation occurred
  end: number;

  // the difference between start and end
  duration: number;

  // reason for cancellation
  cancellationReason: CancellationReason;

  // Optional type of event that triggered cancellation
  eventType?: string;

  // Optional target of event that triggered cancellation
  eventTarget?: EventTarget;

  // the most recent visual update; this can be either a mutation or a load event target
  lastVisibleChange?: HTMLElement | TimestampedMutationRecord;

  navigationType: NavigationType;
};

export enum CancellationReason {
  // navigation has occurred
  NEW_NAVIGATION = 'NEW_NAVIGATION',

  // page was put in background
  VISIBILITY_CHANGE = 'VISIBILITY_CHANGE',

  // user interaction occurred
  USER_INTERACTION = 'USER_INTERACTION',

  // measurement was cancelled because a new one was started
  NEW_MEASUREMENT = 'NEW_MEASUREMENT',

  // manual cancellation via API happened
  MANUAL_CANCELLATION = 'MANUAL_CANCELLATION',
}

`TtvcOptions`

export type TtvcOptions = {
  // decide whether to log debug messages
  debug?: boolean;

  // the duration in ms to wait before declaring the page completely idle
  idleTimeout?: number;

  // a duration in ms to wait before assuming that a single network request
  // was not instrumented correctly
  networkTimeout?: number;
};

Functions

`init()`

type init = (options?: TtvcOptions) -> void;

Sets up instrumentation for the current page and begins monitoring. For the most accurate results, call this as early in pageload as possible.

Accepts an optional options argument (see above).

`start()`

type start = () -> void;

Start a new TTVC measurement.

You do not need to call this for the initial page load. Use this to notify @dropbox/ttvc that a new client-side navigation is about to take place.

`onTTVC()`

type onTTVC = (subscriber: (metric: Metric) -> void) -> () => void;

Register a callback function as a subscriber to new TTVC metric measurements. Returns an "unsubscribe" function which may be called to unregister the subscribed callback function.

The callback function may be called more than once if in-page navigation occurs.

onTTVC may be called more than once to register more than one subscriber.

`cancel()`

type cancel = (eventType?: string) => void;

Abort the current TTVC measurement.

This method is provided as an escape hatch. Consider using cancel to notify @dropbox/ttvc that a user interaction has occurred and conti

Ttvc

Install / Use

README