Record & Tuple Polyfill

The Record and Tuple ECMAScript proposal introduces new deeply immutable value types to JavaScript that have similar access idioms to objects and arrays.

This is an experimental and explicitly not production ready polyfill for the Record and Tuple proposal, and a babel transform to support using the literal syntax.

The polyfill and transform are constant works in progress and are not the source of truth for the proposal.

Requirements

In order to use the syntax transform, babel must be installed with at least version 7.9.0.

In order to use the polyfill, the environment must support WeakMap, WeakRef, and FinalizationRegistry. If implementations of these features are not provided, an error will be thrown.

Installation

To install the transform and polyfill:

# install the babel transform as a dev dependency, only needed at compile time
npm install -D @babel/plugin-proposal-record-and-tuple

# install the polyfill as a regular dependency, needed at runtime
npm install --save @bloomberg/record-tuple-polyfill

Next, add the plugin to your babel configuration. Example:

{
    "plugins": [["@babel/plugin-proposal-record-and-tuple", { "syntaxType": "hash" }]]
}

Note, the syntaxType option is required, and must be set to either hash or bar.

Usage

If the babel transform and the polyfill are setup, you can start using the Record and Tuple literal syntax.

console.log(#{ a: 1 } === #{ a: 1 });
console.log(#[1, 2, 3] === #[1, 2, 3]);

If you want to use the Record or Tuple global objects, you can import them from the polyfill directly.

import { Record, Tuple } from "@bloomberg/record-tuple-polyfill";

const record = Record({ a: 1 });
const tuple = Tuple(1, 2, 3);
const array = [1,2,3];

console.log(Record.isRecord(record));
console.log(Record.keys(record));
console.log(Tuple.from(array));

Unsupported Features

typeof will return an incorrect value when provided a Record or Tuple. Storing +/-0 differs from the spec, Object.is( #[+0][0], #[-0][0] ) returns true when it should return false. This is because the polyfill implements the proposal via interning frozen objects.

Playground

The Record and Tuple polyfill has been deployed in an easy to use REPL here.

Supported Environments/Browsers

The Record and Tuple polyfill requires several JavaScript features that are only available experimentally in browsers, specifically WeakRef and FinalizationRegistry.

The following environments support these experimental features out-of-the-box.

environment	supported
Chrome Canary	✔️

In order to use these experimental features other browsers, you must run the browser/environment with specific flags:

environment	flags
node	--harmony-weak-refs
Chrome	--js-flags="--harmony-weak-refs"
Firefox Nightly	javascript.options.experimental.weakrefs