TypeScript - Indexable Type

Question

TypeScript - Indexable Type

Here is an explanation of some interesting syntax examples:

interface StringArray {
    [index: number]: string;
}

This code snippet defines a type called StringArray, specifying that when the array is indexed with a number, it will return a string. For example:

let myArray: StringArray;
myArray = ["Bob", "Fred"];
let myStr: string = myArray[0];

In this case, myArray can only hold values of type string, as defined by the type constraint. Although the index is specified as a number, in JavaScript, it actually treats it as a string.

Now let's take a look at another syntax example:

class Animal{
  name: string;
}

class Dog extends Animal{
  breed: string;
}

interface NotOkay {
  [x: number]: Animal;
  [x: string]: Dog;   
}

The NotOkay syntax specifies what?

Lastly, we have:

interface NumberDictionary {
    [index: string]: number;
    length: number;    
    name: string;      
}

What does NumberDictionary aim to achieve?

Can you identify the errors present in these two snippets leading to error TS2413 and error: TS2411 respectively?

javascript typescript duck-typing

Answer 1

Answer №1

(as pointed out by @betadeveloper, the examples are sourced from the typescript documentation available at https://www.typescriptlang.org/docs/handbook/interfaces.html)

1+3) The main point highlighted in the documentation is the lack of consistency in this declaration.

The declaration might suggest that the following code is valid:

let no: NotOkay

function byString(key: string): Dog {
   return no[key]
}

function byNumber(key: number): Animal {
   return no[key]
}

However, the issue arises when attempting a lookup like no[0], which behaves the same as no['0'] due to JavaScript's behavior. Therefore, calling byString('0') will actually return an Animal, not necessarily a Dog.

To prevent incorrect interpretations like the one above, where the code appears correct but is flawed for specific string keys (those represented as numbers in string format), TypeScript restricts such declarations.

2+3) The syntax [index: string]: number; allows any string to be used as an index in a NumberDictionary with a corresponding numerical value.

Conversely, name: string; denotes that the property name is a string. However, properties can also be accessed using indices, so when executing

let nd: NumberDictionary

let a = nd['name']
let b = nd.name

both variables a and b essentially hold the same information. Nevertheless, while the initial declaration implies that a is a number, the latter suggests that b is a string, resulting in a contradiction and hence disallowing such declarations.

Answer 2

(as pointed out by @betadeveloper, the examples are sourced from the typescript documentation available at https://www.typescriptlang.org/docs/handbook/interfaces.html)

1+3) The main point highlighted in the documentation is the lack of consistency in this declaration.

The declaration might suggest that the following code is valid:

let no: NotOkay

function byString(key: string): Dog {
   return no[key]
}

function byNumber(key: number): Animal {
   return no[key]
}

However, the issue arises when attempting a lookup like no[0], which behaves the same as no['0'] due to JavaScript's behavior. Therefore, calling byString('0') will actually return an Animal, not necessarily a Dog.

To prevent incorrect interpretations like the one above, where the code appears correct but is flawed for specific string keys (those represented as numbers in string format), TypeScript restricts such declarations.

2+3) The syntax [index: string]: number; allows any string to be used as an index in a NumberDictionary with a corresponding numerical value.

Conversely, name: string; denotes that the property name is a string. However, properties can also be accessed using indices, so when executing

let nd: NumberDictionary

let a = nd['name']
let b = nd.name

both variables a and b essentially hold the same information. Nevertheless, while the initial declaration implies that a is a number, the latter suggests that b is a string, resulting in a contradiction and hence disallowing such declarations.

TypeScript - Indexable Type

Answer №1

Similar questions

Adding a C# variable to a URL in a Razor view using jQuery

Creating a duplicate or copy of an element in jQuery using

Issue encountered while attempting to create entity in jhipster

Tips on converting comma-separated values into HTML table rows using <tr> tags

Issue with Firefox pageMod addon: window.location not functioning properly

What is the process for assigning a random string value to each document within a mongodb collection using the mongo shell?

Trouble with updating a variable within a loop in Cypress

Is there a way to "stream" a specific part of my website to a separate window?

Challenge faced: Angular array variable not refreshing

What is the best way to monitor changes in objects within a JavaScript array?

Testing API route handlers function in Next.js with Jest

The HTML element <a> can have both a href attribute and a onclick attribute

The resume button is failing to activate any functions

Could it be that my function is not returning the correct type?

Discover and update values in JSON using JavaScript

The Node.js callback is executed before the entire function has finished executing

Trying to decide between using a Javascript or HTML5 PDF Editor?

Vue's watch feature will not execute if the watched property remains unchanged during a page reload

NGRX reducer avoids generating errors due to incorrect assignments

How can I move a TableItem from one material UI component to another using drag-and-drop?