Enums vs Constants

Angular: The Full Gamut Edition

Charlie Greenman
August 30, 2020
3 min read

When working with Typescript, which if you are using Angular, then you most definitely are using Typescript. Care must be taken to look into all the nuances that Typescript can offer.

In a non Typescript Setting

In order to define a constant in a non-Typescript setting, we use the const declaration to define variables:

  const UP = "UP";
  const DOWN = "DOWN";
  const LEFT = "LEFT";
  const RIGHT = "RIGHT";

Enums an Introduction

Simply put, Enums allow us to define a set of named constants^1.

  enum PlaneActionTypes {
      Up = "[Plane] Up",
      Down = "[Plane] DOWN",
      Left = "[Plane] LEFT",
      Right = "[Plane] RIGHT",
  }

Benefit of Enums over Constants

Enums allow us to organize a collection of related values. Think of them as a class for values, wherein the value can only be a string , or number.

Current quirk of String Enums

String Enums, as opposed to number Enums, have to be constant initialized with a string literal. To clarify, you might want expect the following to work:

  const prefix = '[Button]'
  enum Direction {
      Up = `${prefix} UP`,
      Down = `${prefix} DOWN`,
      Left = `${prefix} LEFT`,
      Right = `${prefix} RIGHT`,
  }

However, this does not work , because this is not a string literal, i.e. string only.

Convention as a Result of Quirk

As a result of quirk, we need a way of specifying that this action is happening in relation to a specific object. Even though we do have a set using enums, when identifying the string on it's own, from a state management (dev tool) perspective, or console perspective, it will be beneficial to have the string literal, be explicit on it's own.. Please reference above section, "Enums as an Introduction", for how this translates to code in principle.

Side Note - Why No All Caps in Enums?

A const in Javascript can actually be re-assigned to something else. For instance:

const PLANE = 'blackbird';
PLANE = 'thunderbird';
// barf

It is therefore a good convention when using a const, to put it in all caps, when the value is not attend to be re-assigned such as:

const PLANE = 'blackbird';
// woh, I was about to re-assign plane to thunderbird for some weird reason, but
// then I saw PLANE in all caps, so I didn't do it

However, this is not the convention with Enums, of course, because all enums are never re-assigned. It is therefore not necessary to to write in all caps.

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