Can you explain the significance of using square brackets in the typescript enum declaration?

Question

Can you explain the significance of using square brackets in the typescript enum declaration?

While reviewing a typescript file within an Angular ngrx project titled collection.ts, I came across the declaration of enum constants.

import { Action } from '@ngrx/store';
import { Book } from '../models/book';

export enum CollectionActionTypes {
  AddBook = '[Collection] Add Book',
  AddBookSuccess = '[Collection] Add Book Success',
  AddBookFail = '[Collection] Add Book Fail',
  RemoveBook = '[Collection] Remove Book',
  RemoveBookSuccess = '[Collection] Remove Book Success',
  RemoveBookFail = '[Collection] Remove Book Fail',
  Load = '[Collection] Load',
  LoadSuccess = '[Collection] Load Success',
  LoadFail = '[Collection] Load Fail',
}

/**
 * Actions for Adding Books to Collection
 */
export class AddBook implements Action {
  readonly type = CollectionActionTypes.AddBook;

  constructor(public payload: Book) {}
}

...

export type CollectionActions =
  | AddBook
  | AddBookSuccess
  ...
  | LoadFail;

It seems that providing values to enum constants is standard practice, but there's confusion about the significance of [Collection] attached to each constant. Does this extra notation affect the value or convey something else? Any insights on this concept would be greatly appreciated.

angular typescript enums ngrx ngrx-store

Answer 1

Answer №1

Perhaps you are utilizing a flux/redux framework such as ngrx store or something similar. This enumeration defines actions and guarantees that your action keys (which are strings) are distinct on a universal level because ultimately, they all come together into one extensive set of actions that reducers respond to. To maintain this requirement, it is common practice to prefix the type that the actions belong to at the start of the key within square brackets. This is simply a naming convention unrelated to typescript.

For instance,

Let's say you have entities named Book and Category. Both of them could have an action with the key Load Entity. To differentiate between these two action keys, one approach is to include identifiers like [Book] Load Entity and [Category] Load Entity.

Answer 2

Perhaps you are utilizing a flux/redux framework such as ngrx store or something similar. This enumeration defines actions and guarantees that your action keys (which are strings) are distinct on a universal level because ultimately, they all come together into one extensive set of actions that reducers respond to. To maintain this requirement, it is common practice to prefix the type that the actions belong to at the start of the key within square brackets. This is simply a naming convention unrelated to typescript.

For instance,

Let's say you have entities named Book and Category. Both of them could have an action with the key Load Entity. To differentiate between these two action keys, one approach is to include identifiers like [Book] Load Entity and [Category] Load Entity.

Answer 3

Answer №2

The symbols enclosed within the value string do not hold any inherent significance in the context of the enum or TypeScript. In this particular project, the usage of brackets is merely a customary practice - they could easily be replaced by parentheses or braces without affecting the functionality. These symbols may or may not serve another purpose.

You might be reminded of a similar syntax involving computed keys, such as:

const propNam = 'test';
const obj = {[propName]: 2};
console.log(obj.test); // 2

However, such constructions are not permitted in enum declarations. The compiled JavaScript code resulting from an enum declaration does employ computed keys for reverse mappings, as elaborated on in this page:

TypeScript:

enum Enum {
    A
}
let a = Enum.A;
let nameOfA = Enum[a]; // "A"

JavaScript:

var Enum;
(function (Enum) {
    Enum[Enum["A"] = 0] = "A";
})(Enum || (Enum = {}));
var a = Enum.A;
var nameOfA = Enum[a]; // "A"

It's important to note that this concept differs significantly from the use of brackets in your scenario.

Answer 4

The symbols enclosed within the value string do not hold any inherent significance in the context of the enum or TypeScript. In this particular project, the usage of brackets is merely a customary practice - they could easily be replaced by parentheses or braces without affecting the functionality. These symbols may or may not serve another purpose.

You might be reminded of a similar syntax involving computed keys, such as:

const propNam = 'test';
const obj = {[propName]: 2};
console.log(obj.test); // 2

However, such constructions are not permitted in enum declarations. The compiled JavaScript code resulting from an enum declaration does employ computed keys for reverse mappings, as elaborated on in this page:

TypeScript:

enum Enum {
    A
}
let a = Enum.A;
let nameOfA = Enum[a]; // "A"

JavaScript:

var Enum;
(function (Enum) {
    Enum[Enum["A"] = 0] = "A";
})(Enum || (Enum = {}));
var a = Enum.A;
var nameOfA = Enum[a]; // "A"

It's important to note that this concept differs significantly from the use of brackets in your scenario.

Can you explain the significance of using square brackets in the typescript enum declaration?

Answer №1

Answer №2

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