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.
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
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);
});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 = [];
}
});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,
});
});@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>
);
};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';
};
};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;
};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).
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.
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.
type cancel = () => 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 continuing the measurement may produce an invalid result.
type incrementAjaxCount = () -> void;
type decrementAjaxCount = () -> void;Use these functions to instrument AJAX requests in your application. Try to ensure incrementAjaxCount and decrementAjaxCount are called exactly once for each request, regardless of success or failure.
e.g.
// patch window.fetch
const nativeFetch = window.fetch;
window.fetch = (...args) => {
TTVC.incrementAjaxCount();
return nativeFetch(...args).finally(TTVC.decrementAjaxCount);
};@dropbox/ttvc relies on the following browser features.
See browsers that support all above features on caniuse.com:
If you would like to use this library and support a browser that does not support one of these features, you will be responsible for supplying polyfills and/or recompiling the library to remove unsupported syntax.
Browsers today don't automatically report much about their paint cycles. It would definitely be an improvement if browser developers decided to report this value for us.
In the meantime, we approximate TTVC using a heuristic based on a combination of observing mutations and "load" events.
For a more detailed write-up of the architecture, check out our announcement blog post!
https://dropbox.tech/frontend/measuring-ttvc-web-performance-metric-open-source-library
To develop this package, you will need node version 16 or greater, and the yarn package manager. Once you have these prerequisites, install project dependencies with:
yarn install
This project is developed with TypeScript. You can compile the TypeScript source files to JavaScript with:
$ yarn build
While testing locally, you may find it useful to build the rollup bundle in watch mode.
$ yarn build:rollup --watch
You can run all tests together with:
$ yarn test
There are four individual test scripts
$ yarn test:lint // runs eslint
$ yarn test:typecheck // runs typescript with --noEmit
$ yarn test:unit // runs jest unit tests
$ yarn test:e2e // runs playwright tests (requires yarn build to have been run)
Before running any playwright tests, you will need to install the default set of browsers:
$ npx playwright install
Run test suite:
$ yarn build // build the ttvc package before testing
$ yarn test:e2e
To manually test pages, start the test server.
$ yarn express
You can launch tests in the browser by opening http://localhost:3000/test/[test-folder]. e.g. http://localhost:3000/test/ajax1/