Camille Hodoul

Flow types for Immutable.js Records

March 08, 2020

What is a Record?

An Immutable.Record instance is an object with a defined set of keys and default values.
They are instanciated by factories, which are created by calling Immutable.Record({ ...defaultValues }).

Learn more here.

(For the sake of brevity, the following mentions of Record and Map assume the Immutable. namespace)

Records: when and why?


  • you have a Map with string keys and
  • you wish you had some kind of garantees about which keys you can expect and
  • you wish you had some safety about the types of the values of those keys and
  • you don’t want to write tens of lines of defensive programming,

consider using a Record.

One such case could be the state in a React application.

Typing Records in Flow

If you’re typing your JavaScript with Flow, you have to explain it what a Record is.
Luckily, Immutable.js 4 provides 2 useful types: RecordOf and RecordFactory.


import type { RecordOf, RecordFactory } from "immutable";
import { Record, List } from "immutable";

 * Describe the shape of your object
type MyObject = {|
    key1: string,
    key2: number,
    key3: boolean,
    key4: List<string>,

type MyRecord = RecordOf<MyObject>;

 * Create the factory with the default values
const makeRecord: RecordFactory<MyObject> = Record({
    key1: "defaultString",
    key2: 1,
    key3: false,
    key4: List()

// create an instance with default values
const myRecordInstance = makeRecord();
// Inference works as expected
myRecordInstance.get("key1"); // Works
myRecordInstance.get("wrongKey"); // Error

// create an instance from an object
const myOtherInstance = makeRecord({
    key1: "My string",
    key4: List(["a", "b", "c"])
myOtherInstance.get("key4").toSet(); // Works
myOtherInstance.get("key2"); // Works

The garantees you’ll get:

  • MyRecord instances will always have the expected keys,
  • they won’t have unexpected keys,
  • the values will always have the expected type.

I'm a JavaScript and PHP developer living in Grenoble, France.
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