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JavaScript User Guide

Perspective's JavaScript library offers a configurable UI powered by a fast streaming data engine. Developers are able to pick and choose the modules they require for their use case, and users are presented with a clean user interface through which to analyze data.

More Examples are available on GitHub.


Perspective releases contain several different builds for easy usage in most environments, either via NPM with or without a bundler, or via <script> tag from a CDN or asset server of your choice. Depending on which build you choose, due to the presence of both WebAssembly and WebWorkers, the installation process for Perspective may be somewhat more complex than most "pure" Javascript libraries if you want to achieve optimal initial load-time performance.

From NPM (Node.js)

To use Perspective from a Node.js server, simply install via NPM.

$ yarn add @finos/perspective

From NPM (Browser)

For using Perspective as a dependency in a webpack (or other bundler) app, Perspective's WebAssembly data engine is available via NPM in the same package, @finos/perspective. For the @finos/perspective-viewer UI, a few additional packages are required:

$ yarn add @finos/perspective @finos/perspective-viewer @finos/perspective-viewer-d3fc @finos/perspective-viewer-datagrid

Perspective requires the browser to have access to Perspective's .worker.js and .wasm assets in addition to the bundled .js scripts. By default, Perspective inlines these assets into the .js scripts, and delivers them in one file. This has no runtime performance impact, but does increase asset load time. Most apps should make use of @finos/perspective-webpack-plugin which will package these files correctly form your existing Webpack configuration.

Webpack Plugin (optional)

When importing perspective from NPM modules for a browser application, you should use @finos/perspective-webpack-plugin to manage the .worker.js and .wasm assets for you. Doing so will improve your application's initial load performance, the plugin-compiled version of Perspective:

  • Downloads .wasm and .js assets in parallel.
  • Compiles .wasm incrementally via streaming instantiation.
  • Lazily downloads large features only when used such as monaco-editor.
  • overall bundle size is ~20% smaller (due to bas64 encoding overhead).

The plugin handles downloading and packaging Perspective's additional assets, and is easy to set up in your webpack.config:

const PerspectivePlugin = require("@finos/perspective-webpack-plugin");

module.exports = {
entry: "./in.js",
output: {
filename: "out.js",
path: "build",
plugins: [new PerspectivePlugin()],

@finos/perspective-viewer has a dependence on monaco-editor, which itself depends on several CSS assets. If your webpack config uses a loader for "*.css" or similar, you may need to exclude monaco-editor from this loader to prevent double-encoding:

module.exports = {
// ...

module: {
rules: [
test: /\.css$/,
exclude: [/monaco-editor/], // <- Exclude `monaco-editor`
use: [{loader: "style-loader"}, {loader: "css-loader"}],

From CDN

Perspective can be loaded directly from most CDNs, such as, which is the easiest way to get started with Perspective in the browser, and perfect for spinning up quick instances of perspective-viewer without installing or bundling. There are two supported builds you may use, a UMD build and a type="module" ESM build.

While CDNs are great for development builds and small apps, for production usage you should incorporate Perspective into your application with a bundler like Webpack, described above.


This build is equivalent to the inline build described above, and contains all JavaScript, CSS, WebAssembly and WebWorker assets bundled in a single .js file. To use the UMD build from a, add these scripts to your .html's <head> section:

<script src=""></script>
<script src=""></script>
<script src=""></script>
<script src=""></script>


Once added to your page, you can access the engine's JavaScript API through the perspective symbol and the browser's Custom Elements API:

const worker = window.perspective.worker();
const table = await worker.table({ A: [1, 2, 3] });
const view = await table.view({ sort: [["A", "desc"]] });

const viewer = document.createElement("perspective-viewer");


This build separates out Perspective's JavaScript, WebAssembly and various assets into individual files, allowing the browser to load them lazily, in parallel or not at all if needed. To use this build, you must include the perspective asset files in a script tag with the type="module" attribute set.



When using the ESM build, there is no global perspective symbol, so you must import the @finos/perspective module in a type="module" script as well:

<script type="module">
import perspective from "";

const worker = perspective.worker();
const table = agent.table({x: [1, 2, 3, 4, 5]});

Module Structure

Perspective is designed for flexibility, allowing developers to pick and choose which modules they need for their specific use case. The main modules are:

  • @finos/perspective
    The data engine library, as both a browser ES6 and Node.js module. Provides a WebAssembly, WebWorker (browser) and Process (node.js) runtime.

  • @finos/perspective-viewer
    A user-configurable visualization widget, bundled as a Web Component. This module includes the core data engine module as a dependency.

<perspective-viewer> by itself only implements a trivial debug renderer, which prints the currently configured view() as a CSV. Plugin modules for popular JavaScript libraries, such as d3fc, are packaged separately and must be imported individually.

Perspective offers these plugin modules:

  • @finos/perspective-viewer-datagrid
    A custom high-performance data-grid component based on HTML <table>.

  • @finos/perspective-viewer-d3fc
    A <perspective-viewer> plugin for the d3fc charting library.

When imported after @finos/perspective-viewer, the plugin modules will register themselves automatically, and the renderers they export will be available in the plugin dropdown in the <perspective-viewer> UI.

Which modules should I import?

Depending on your requirements, you may need just one, or all, Perspective modules. Here are some basic guidelines to help you decide what is most appropriate for your project:

  • For Perspective's high-performance streaming data engine (in WebAssembly), or for a purely Node.js based application, import:

    • @finos/perspective, as detailed here
  • For Perspective as a simple, browser-based data visualization widget, you will need to import:

    • @finos/perspective, detailed here
    • @finos/perspective-viewer, detailed here
    • @finos/perspective-viewer-datagrid for data grids
    • @finos/perspective-viewer-d3fc for charting
  • For more complex cases, such as sharing tables between viewers and binding a viewer to a remote view in Node.js, you will likely need all Perspective modules.

perspective data engine library

As a library, perspective provides a suite of streaming pivot, aggregate, filter and sort operations for tabular data. The engine can be instantiated in process or in a Web Worker (browser only); in both cases, perspective exports a nearly identical API.

It exports Perspective's data interfaces:

  • table(): an interface over a single dataset, used to input static and streaming data into Perspective.
    • In the browser, table()s live in a Web Worker to isolate their runtime from the renderer.
  • view(): a continuous query of a table(), used to read data and calculate analytics from a table().
    • view()s also live in a Web Worker when used in a browser.
    • A single table() may have many view()s attached at once.

@finos/perspective also exports process management functions, such as worker() and websocket() (in the browser) and WebSocketServer() (in Node.js). See the API documentation for a complete reference on all exported methods. This module is a dependency of @finos/perspective-viewer, and is not needed if you only intend to use <perspective-viewer> to visualize simple data.

Importing in the browser

perspective can be imported as an ES6 module and/or require syntax if you're using a bundler such as Webpack (and the @finos/perspective-webpack-plugin):

import perspective from "@finos/perspective";


const perspective = require("@finos/perspective");

@finos/perspective also comes with a pre-built bundle which exports the global perspective module name in vanilla JavaScript, when e.g. importing via a CDN:

<script src="@finos/perspective"></script>

... or as a module:

<script type="module" src="@finos/perspective/dist/cdn/perspective.js"></script>

Instantiating a new worker()

Once imported, you'll need to instantiate a perspective engine via the worker() method. This will create a new Web Worker (browser) or Process (Node.js) and load the WebAssembly binary; all calculation and data accumulation will occur in this separate process.

const worker = perspective.worker();

The worker symbol will expose the full perspective API for one managed Web Worker process. You are free to create as many as your browser supports, but be sure to keep track of the worker instances themselves, as you'll need them to interact with your data in each instance.

Importing in Node.js

The Node.js runtime for the @finos/perspective module runs in-process by default and does not implement a child_process interface. Hence, there is no worker() method, and the module object itself directly exports the full perspective API.

const perspective = require("@finos/perspective");

In Node.js, perspective does not run in a WebWorker (as this API does not exist in Node.js), so no need to call the .worker() factory function - the perspective library exports the functions directly and run synchronously in the main process.

Serializing data using to_*()

The view() allows for serialization of data to the user through the to_json(), to_columns(), to_csv(), and to_arrow() methods. These methods return a promise for the calculated data:

Via Promise

// an array of objects representing each row
view.to_json().then((json) => console.log(json));

// an object of arrays representing each column
view.to_columns().then((json) => console.log(json));

// a CSV-formatted string
view.to_csv().then((csv) => console.log(csv));

// an Arrow binary serialized to ArrayBuffer
view.to_arrow().then((arrow) => console.log(arrow));

Via ES6 await/async

async function print_data() {
console.log(await view.to_json());
console.log(await view.to_columns());
console.log(await view.to_csv());
console.log(await view.to_arrow());

Deleting a table() or view()

Unlike standard JavaScript objects, Perspective objects such as table() and view() store their associated data in the WebAssembly heap. Because of this, as well as the current lack of a hook into the JavaScript runtime's garbage collector from WebAssembly, the memory allocated to these Perspective objects does not automatically get cleaned up when the object falls out of scope.

In order to prevent memory leaks and reclaim the memory associated with a Perspective table() or view(), you must call the delete() method:

await view.delete();

// This method will throw an exception if there are still `view()`s depending
// on this `table()`!
await table.delete();

perspective-viewer web component library

<perspective-viewer> provides a complete graphical UI for configuring the perspective library and formatting its output to the provided visualization plugins.

If you are using webpack or another bundler which supports ES6 modules, you only need to import the perspective-viewer libraries somewhere in your application - these modules export nothing, but rather register the components for use within your site's regular HTML:

import "@finos/perspective-viewer";
import "@finos/perspective-viewer-datagrid";
import "@finos/perspective-viewer-d3fc";

Once imported, the <perspective-viewer> Web Component will be available in any standard HTML on your site. A simple example:

<perspective-viewer id="view1"></perspective-viewer>


const viewer = document.createElement("perspective-viewer");


Theming is supported in perspective-viewer and its accompanying plugins. A number of themes come bundled with perspective-viewer; you can import any of these themes directly into your app, and the perspective-viewers will be themed accordingly:

// Themes based on Google's Material Design Language
import "@finos/perspective-viewer/dist/css/material.css";
import "@finos/perspective-viewer/dist/css/material-dark.css";

// Other themes
import "@finos/perspective-viewer/dist/css/solarized.css";
import "@finos/perspective-viewer/dist/css/solarized-dark.css";
import "@finos/perspective-viewer/dist/css/monokai.css";
import "@finos/perspective-viewer/dist/css/vaporwave.css";

Alternatively, you may use themes.css, which bundles all default themes

import "@finos/perspective-viewer/dist/css/themes.css";

If you choose not to bundle the themes yourself, they are available through CDN. These can be directly linked in your HTML file:


Note the crossorigin="anonymous" attribute. When including a theme from a cross-origin context, this attribute may be required to allow <perspective-viewer> to detect the theme. If this fails, additional themes are added to the document after <perspective-viewer> init, or for any other reason theme auto-detection fails, you may manually inform <perspective-viewer> of the available theme names with the .resetThemes() method.

// re-auto-detect themes

// Set available themes explicitly (they still must be imported as CSS!)
viewer.resetThemes(["Material Light", "Material Dark"]);

<perspective-viewer> will default to the first loaded theme when initialized. You may override this via .restore(), or provide an initial theme by setting the theme attribute:

<perspective-viewer theme="Material Light"></perspective-viewer>


const viewer = document.querySelector("perspective-viewer");
viewer.restore({theme: "Material Dark"});

Loading data into <perspective-viewer>

Data can be loaded into <perspective-viewer> in the form of a Table() or a Promise<Table> via the load() method.

// Create a new worker, then a new table promise on that worker.
const table = await perspective.worker().table(data);

// Bind a viewer element to this table.

Sharing a table() between multiple perspective-viewers

Multiple perspective-viewers can share a table() by passing the table() into the load() method of each viewer. Each perspective-viewer will update when the underlying table() is updated, but table.delete() will fail until all perspective-viewer instances referencing it are also deleted:

const viewer1 = document.getElementById("viewer1");
const viewer2 = document.getElementById("viewer2");

// Create a new WebWorker
const worker = perspective.worker();

// Create a table in this worker
const table = await worker.table(data);

// Load the same table in 2 different <perspective-viewer> elements

// Both `viewer1` and `viewer2` will reflect this update
table.update([{x: 5, y: "e", z: true}]);

Server-only via WebSocketServer() and Node.js

For exceptionally large datasets, a <perspective-viewer> can be bound to a perspective.table() instance running in Node.js remotely, rather than creating one in a Web Worker and downloading the entire data set. This trades off network bandwidth and server resource requirements for a smaller browser memory and CPU footprint.

In Node.js:

const {WebSocketServer, table} = require("@finos/perspective");
const fs = require("fs");

// Start a WS/HTTP host on port 8080. The `assets` property allows
// the `WebSocketServer()` to also serves the file structure rooted in this
// module's directory.
const host = new WebSocketServer({assets: [__dirname], port: 8080});

// Read an arrow file from the file system and host it as a named table.
const arr = fs.readFileSync(__dirname + "/superstore.arrow");
table(arr).then((table) => {
host.host_table("table_one", table);

In the browser:

const elem = document.getElementsByTagName("perspective-viewer")[0];

// Bind to the server's worker instead of instantiating a Web Worker.
const websocket = perspective.websocket(
window.location.origin.replace("http", "ws")

// Bind the viewer to the preloaded data source. `table` and `view` objects
// live on the server.
const server_table = await websocket.open_table("table_one");

// Or load data from a table using a view. The browser now also has a copy of
// this view in its own `table`, as well as its updates transferred to the
// browser using Apache Arrow.
const worker = perspective.worker();
const server_view = await server_table.view();
const client_table = worker.table(server_view);

<perspective-viewer> instances bound in this way are otherwise no different than <perspective-viewer>s which rely on a Web Worker, and can even share a host application with Web Worker-bound table()s. The same promise-based API is used to communicate with the server-instantiated view(), only in this case it is over a websocket.

Server-only via perspective-python and Tornado

perspective-python is designed to be cross-compatible with the perspective and perspective-viewer libraries. Similar to WebsocketServer in Node.js, perspective-python runs on the server without any memory limits, reducing resource usage in the browser. For more detailed documentation on the Python API, see the Python user guide or the Python API documentation.

The simplest implementation uses Tornado as a websocket server in Python, hosting an endpoint at which a table() can be accessed:

from perspective import Table, PerspectiveManager, PerspectiveTornadoHandler

# Create an instance of PerspectiveManager, and host a Table
MANAGER = PerspectiveManager()
TABLE = Table(large_dataset)

# The Table is exposed at `localhost:8888/websocket` with the name `data_source`
MANAGER.host_table("data_source", TABLE)

app = tornado.web.Application([
(r"/", MainHandler),
# create a websocket endpoint that the client JavaScript can access
(r"/websocket", PerspectiveTornadoHandler, {"manager": MANAGER, "check_origin": True})

# Start the Tornado server
loop = tornado.ioloop.IOLoop.current()

The JavaScript implementation of this does not require Webpack or any bundler, and can be achieved in a single HTML file:


<perspective-viewer id="viewer" editable></perspective-viewer>

window.addEventListener("DOMContentLoaded", async function () {
// Create a client that expects a Perspective server
// to accept connections at the specified URL.
const websocket = perspective.websocket(

/* `table` is a proxy for the `Table` we created on the server.

All operations that are possible through the JavaScript API are possible
on the Python API as well, thus calling `view()`, `schema()`, `update()`
etc. on `const table` will pass those operations to the Python `Table`,
execute the commands, and return the result back to JavaScript.*/
const table = websocket.open_table("data_source_one");

// Load this in the `<perspective-viewer>`.

Any operation performed on the <perspective-viewer> instance or on table will be forwarded to Python, which will execute the operation and return the results back to JavaScript.

Persistent <perspective-viewer> configuration via save()/restore().

<perspective-viewer> is persistent, in that its entire state (sans the data itself) can be serialized or deserialized. This include all column, filter, pivot, expressions, etc. properties, as well as datagrid style settings, config panel visibility, and more. This overloaded feature covers a range of use cases:

  • Setting a <perspective-viewer>'s initial state after a load() call.
  • Updating a single or subset of properties, without modifying others.
  • Resetting some or all properties to their data-relative default.
  • Persisting a user's configuration to localStorage or a server.

Serializing and deserializing the viewer state

To retrieve the entire state as a JSON-ready JavaScript object, use the save() method. save() also supports a few other formats such as "arraybuffer" and "string" (base64, not JSON), which you may choose for size at the expense of easy migration/manual-editing.

const json_token = await;
const string_token = await"string");

For any format, the serialized token can be restored to any <perspective-viewer> with a Table of identical schema, via the restore() method. Note that while the data for a token returned from save() may differ, generally its schema may not, as many other settings depend on column names and types.

await elem.restore(json_token);
await elem.restore(string_token);

As restore() dispatches on the token's type, it is important to make sure that these types match! A common source of error occurs when passing a JSON-stringified token to restore(), which will assume base64-encoded msgpack when a string token is used.

// This will error!
await elem.restore(JSON.stringify(json_token));

Updating individual properties

Using the JSON format, every facet of a <perspective-viewer>'s configuration can be manipulated from JavaScript using the restore() method. The valid structure of properties is described via the ViewerConfig and embedded ViewConfig type declarations, and View chapter of the documentation which has several interactive examples for each ViewConfig property.

// Set the plugin (will also update `columns` to plugin-defaults)
await elem.restore({plugin: "X Bar"});

// Update plugin and columns (only draws once)
await elem.restore({plugin: "X Bar", columns: ["Sales"]});

// Open the config panel
await elem.restore({settings: true});

// Create an expression
await elem.restore({
columns: ['"Sales" + 100'],
expressions: ['"Sales" + 100'],

// ERROR if the column does not exist in the schema or expressions
// await elem.restore({columns: ["\"Sales\" + 100"], expressions: []});

// Add a filter
await elem.restore({filter: [["Sales", "<", 100]]});

// Add a sort, don't remove filter
await elem.restore({sort: [["Prodit", "desc"]]});

// Reset just filter, preserve sort
await elem.restore({filter: undefined});

// Reset all properties to default e.g. after `load()`
await elem.reset();

Another effective way to quickly create a token for a desired configuration is to simply copy the token returned from save() after settings the view manually in the browser. The JSON format is human-readable and should be quite easy to tweak once generated, as save() will return even the default settings for all properties. You can call save() in your application code, or e.g. through the Chrome developer console:

// Copy to clipboard
copy(await document.querySelector("perspective-viewer").save());

Update events

Whenever a <perspective-viewer>s underlying table() is changed via the load() or update() methods, a perspective-view-update DOM event is fired. Similarly, view() updates instigated either through the Attribute API or through user interaction will fire a perspective-config-update event:

elem.addEventListener("perspective-config-update", function (event) {
var config =;
console.log("The view() config has changed to " + JSON.stringify(config));

Click events

Whenever a <perspective-viewer>'s grid or chart is clicked, a perspective-click DOM event is fired containing a detail object with config, column_names, and row.

The config object contains an array of filters that can be applied to a <perspective-viewer> through the use of restore() updating it to show the filtered subset of data.

The column_names property contains an array of matching columns, and the row property returns the associated row data.

elem.addEventListener("perspective-click", function (event) {
var config = event.detail.config;