As I was exploring some old code, I came across the following snippet:
let isNew: boolean = null
let myName: string = undefined
This snippet seems to indicate that in typescript, a variable of type boolean can accept null and a variable of type string can accept undefined.
Could this mean that null and undefined are considered subtypes of other types in typescript?
If I am mistaken, please correct me. Any suggestions or clarifications would be greatly appreciated!
If you open up a javascript console and type typeof(null) along with typeof(undefined), you'll see that TypeScript treats them the same way since ultimately TypeScript code is converted to javascript.
In TypeScript, null is considered a type of object, while undefined is classified as a type of undefined itself.
undefined is the value automatically assigned to any declared variable in javascript that has not been given a value. On the other hand, null is purposely assigned as a value, but it's also possible to assign a variable to undefined intentionally.
The code snippet you provided would likely result in errors when compiled by most configurations of the TypeScript compiler, depending on the settings in the tsconfig.json file. To declare a variable in TypeScript of type T that can be either null or undefined, where T represents another type (such as boolean or string), the variable must be defined accordingly using syntax like T|null or T|undefined. For example:
let isNew1: boolean = null; // results in an error
let isNew2: boolean|null = null; // no issues here
let isNew3: boolean|null; // error - cannot assign undefined
let myName1: string = undefined; // leads to an error
let myName2: string|undefined = undefined; // works fine
let myName3: string|undefined; // valid declaration
Additionally, you have the option to allow for undefined values in types and interfaces in TypeScript by utilizing the ? symbol (e.g., myVariable?: string).
I am currently working on developing a user authentication system that includes features for Login, Signup, and Logout. As part of this project, I have created a middleware.ts file in the src directory, which is located next to the app directory. Below is ...
I have implemented highchart for graph rendering and utilized the renderer to draw a custom line within the chart. I am looking for a way to recalculate and repaint this path whenever there is a change in data. The framework being used is highcharts-ng alo ...
When it comes to design editors, there are plenty of options such as Canva, Vistacreate, and Adobe Express. Each one uses a variety of styles for elements/objects/shapes. Are there any databases where I can find these resources? Some popular places include ...
I have a form that allows users to create Todo's. An ITodo object includes the following properties: export interface ITodo { id: number; title: string; priority: ITodoPriority; } export interface ITodoPriority { id: number; name ...
I am interested in creating a custom component using a directive. After reviewing numerous tutorials, I am still finding it confusing. Can someone please explain how a directive works? The custom component I have in mind is: <shout-list></shout-l ...
It's incredible how simple Html5 dialogs are. It feels like they should have been here 15 years ago! I'm having trouble getting ng-show to work with dialogs. Any tips on how to make it happen? <!DOCTYPE html> <html ng-app="project"> ...
I currently have 2 mutations that are functioning well. However, I encountered an issue when attempting to implement onSuccess and invalidateQueries. The error message displayed is as follows: Property 'mutateAsync' does not exist on type '{ ...
I have a survey application, where users can participate in polls by voting for different options. Within the HTML template, I utilize ng-show to display whether a user has voted for a particular option in a poll or if it remains unvoted for the user: < ...
From what I understand, TypeScript was created by Microsoft and is used to dynamically generate JavaScript. I'm curious about the distinctions between TypeScript and AtScript. Which one would be more beneficial for a JavaScript developer to learn? ...
I have the following TypeScript code snippet: interface Stop { code: string } interface FareZone { name: string; stops: Stop[]; } const outbound: FareZone[] = [{name: 'Zone A', stops: [{ code: 'C00'}] }, {name: 'Zone B ...
While experimenting with a force layout, I noticed that when I drag an item aggressively, it sometimes causes everything to freeze. This raises the following questions: What is the reason behind this issue? Is there any way to detect this and restart th ...
Can you explain how the solution discussed in this Stack Overflow thread works? I'm interested in understanding the specific logic behind selecting ID0001 in the code snippet provided below: element.all(by.repeater('item in $data track by $index ...
Is there a way to dynamically add a class to the i tag after a button is clicked in AngularJS? <button type="button" title="Like" ng-click="countLikes(product.title)" class="btn btn-compare"> <i class="fa fa-thumbs-o-up"></i> </ ...
My router configuration: .config(['$routeProvider',function($routeProvider) { $routeProvider .when('/allbooks', { templateUrl: 'find/showAllMy.html', controller: 'controlador' }).when ...
I've been attempting to modify the background color of my Angular Material Dialog by utilizing the panelClass property in the MatDialogConfig. Unfortunately, I'm encountering a partial success. I am aiming to set the background color as red (jus ...
My goal is to create a component property that is read-only. However, I am facing an issue where the readonly modifier does not seem to have any effect. View example on stackblitz According to the documentation, once I initialize the cars property in the ...
I have successfully implemented checklist-model.js for angular to select from a dynamically generated list of objects. However, I now need to create the functionality to move unchecked checkboxes into a new array (and remove them when checked again). Can a ...
Is it possible to detect click events on SVG elements such as path or g using Angular? To see an example of this, check out this demo. Right now, if a (click) event binding is added to the svg elements, the click() event handler will trigger. However, how ...
I have a question about this specific div element: <div class="relative rounded overflow-clip border w-full" [style.aspect-ratio]="media.value.ratio"> <img fill priority [ngSrc]="media.value.src || ftaLogo" ...
What is preventing me from having a binding in a nested object within my scope object, as demonstrated here: app.directive('myDirective', function() { return { scope: { dropdown: { option: '=selectedO ...