typescript 38 lines · 7 steps

A module-scoped Singleton in TypeScript

Module caching guarantees one shared ConfigManager instance, with typed get/set accessors over a private config object.

Explained by highlit
Intermediate singleton generics encapsulation immutability module-scope
1// A module-scoped Singleton: the single instance lives in this module's
2// closure and is never exposed directly.
3 
4interface AppConfig {
5 apiUrl: string;
6 retries: number;
7}
8 
9class ConfigManager {
10 private config: AppConfig;
11 
12 constructor() {
13 this.config = {
14 apiUrl: process.env.API_URL ?? "http://localhost:3000",
15 retries: Number(process.env.RETRIES ?? 3),
16 };
17 }
18 
19 get<K extends keyof AppConfig>(key: K): AppConfig[K] {
20 return this.config[key];
21 }
22 
23 set<K extends keyof AppConfig>(key: K, value: AppConfig[K]): void {
24 this.config[key] = value;
25 }
26 
27 snapshot(): Readonly<AppConfig> {
28 return Object.freeze({ ...this.config });
29 }
30}
31 
32// Instantiated once at module load; module caching guarantees a single
33// instance shared across every importer.
34const instance = new ConfigManager();
35 
36export function getConfigManager(): ConfigManager {
37 return instance;
38}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1ES modules are cached, so a const initialized at load time becomes a natural singleton without any locking.
  2. 2Keying generics on `keyof` ties an accessor's return type to the property you ask for, catching typos at compile time.
  3. 3Returning a frozen shallow copy lets callers read state safely without mutating the source of truth.

Related explainers

typescript 
import {
  CallHandler,
  ExecutionContext,
  Injectable,

Wrapping responses in a NestJS interceptor

interceptors rxjs response-shaping
Intermediate 7 steps
php 
<?php
 
namespace App\Support;
 

Locale-aware formatting with PHP's intl extension

internationalization encapsulation constructor-injection
Intermediate 7 steps
typescript 
type RetryOptions = {
  retries?: number;
  timeoutMs?: number;
  baseDelayMs?: number;

Retry with timeout and backoff in TypeScript

promises retry exponential-backoff
Intermediate 10 steps
php 
<?php
 
namespace App\Support;
 

Merging query params onto a URL in PHP

url-parsing query-strings immutability
Intermediate 8 steps
typescript 
import { Pipe, PipeTransform, ChangeDetectorRef, NgZone, OnDestroy } from '@angular/core';
 
@Pipe({
  name: 'timeAgo',

A self-refreshing timeAgo pipe in Angular

impure-pipe change-detection timers
Advanced 10 steps
go 
package cache
 
import (
	"container/list"

Building a generic LRU cache in Go

lru-cache generics linked-list
Intermediate 8 steps

Share this explainer

Here's the card — post it anywhere.

A module-scoped Singleton in TypeScript — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code