einmal v1.0.2

einmal

A simple function for declaring variables that can be assigned later and only once. This serves few practical purposes, and was mostly a way of getting familiar with the Proxy API.

Installation

npm install einmal

Usage

Creating an einmal works similar to declaring a variable using const, except that it allows for a single reassignment after being declared. It is important to note that einmal does not make the assigned values immutable. It simply prevents replacing the reference to said value - just like how const works.

Basic example

import { einmal } from "einmal";

const thing = einmal();

// first mutation
thing.value = "this is fine"; // all good

// second mutation
thing.value = "this will throw"; // throws

Pitfall - Example with mutable value

const thing = einmal();

thing.value = { name: "Bob", age: 31 }; // so far so good

// also fine, because we're modifying
// a property of our target - we're not
// changing the reference held by our thing
thing.value.name = "Alice";

With types

einmal is written in TypeScript and offers opt-in strong typing for the einmal value.

type Person = {
  name: string;
  age: number;
};

const thing = einmal<Person>();

thing is of type Einmal<Person>. Continued:

console.log(thing.value.age); // error: value might be undefined

thing.value = { name: "John", age: "yes" }; // also error, since age should be a number

einmal?

German: once