Type Your HashMaps with TypeScript

Type Your HashMaps with TypeScript

Rene Rubalcava | December 31, 2020

It's my HashMap, I'll do what I want to

First off, I know what you're going to say. Why use your own HashMap if you can make a Map or WeakMap? Sometimes, you just want to use a Plain Old JavaScript Object (POJO). There could be a variety of reasons for this, maybe you just need to keep track of data when iterating or maybe you're working on some arguments for a function.

function doCoolStuff(params: Params, options: OptionsMap) {
    // cool stuff

In the case above, I might have a strictly typed Params, but the options could vary, so I want to keep them pretty flexible. I use any a lot as a placeholder and try to get away from it when I can. A cool option for this is to type a hashMap.

// simple HashMap, assume it can take all string or boolean values
interface OptionsMap { [key: string]: string | boolean }

const options: OptionsMap = {
    cacheId: 'u21987',
    useDefaultValues: true,
    responseAs: 'json'

Looking at this code, we can type out the options so the values can be either a string or a boolean. This allows users to enter any extra options they want that are supported by this function. This is definitely useful if you work with a development server for your back-end and want to pass special dev options for testing.

Type Happy

You can even be more explicit and type the values to something more concrete.

interface User {
    name: string;
    level: number;
    status: string;

interface HashMap<T> {
    [key: string]: T

// init the map
const userMap: HashMap<User> = {};

userMap['bob'] = {
    name: 'bob',
    level: 12,
    status: 'afk'

Here, we've typed our HashMap to only accept values of T. This is pretty useful when working with response data or possibly some cache implementation you might build.

You can do more

This typed HashMap is a great way to start a building block of types in your application. You might even move on to incorporatinbg Utility Types. You can mix and match these to do some amazing things with TypeScript. I just wanted to cover a really simple way you can get started to move beyond typing your still-in-progress types as any.