typescript - w3toppers.com

How can I listen for keypress event on the whole page?

I would use @HostListener decorator within your component: import { HostListener } from ‘@angular/core’; @Component({ … }) export class AppComponent { @HostListener(‘document:keypress’, [‘$event’]) handleKeyboardEvent(event: KeyboardEvent) { this.key = event.key; } } There are also other options like: host property within @Component decorator Angular recommends using @HostListener decorator over host property https://angular.io/guide/styleguide#style-06-03 @Component({ … host: { … Read more

Typescript Interface – Possible to make “one or the other” properties required?

If you’re truly after “one property or the other” and not both you can use never in the extending type: interface MessageBasics { timestamp?: number; /* more general properties here */ } interface MessageWithText extends MessageBasics { text: string; attachment?: never; } interface MessageWithAttachment extends MessageBasics { text?: never; attachment: string; } type Message = … Read more

Angular 2 beta.17: Property ‘map’ does not exist on type ‘Observable’

You need to import the map operator: import ‘rxjs/add/operator/map’

Purpose of declare keyword in TypeScript

TL;DR declare is used to tell the compiler “this thing (usually a variable) exists already, and therefore can be referenced by other code, also there is no need to compile this statement into any JavaScript”. Common Use Case: You add a reference to your web page to a JavaScript file that the compiler knows nothing … Read more

Are strongly-typed functions as parameters possible in TypeScript?

Sure. A function’s type consists of the types of its argument and its return type. Here we specify that the callback parameter’s type must be “function that accepts a number and returns type any“: class Foo { save(callback: (n: number) => any) : void { callback(42); } } var foo = new Foo(); var strCallback … Read more

Specify method’s type without converting the method to property

There’s an existing suggestion at microsoft/TypeScript#22063 to allow function statements (and presumably method definitions) to be typed using a function/callable type alias or interface. Doesn’t look like there’s much traction there, but if you believe you have a strong use case, you might want to go there and give it a 👍 or comment. In … Read more

Why this CustomExtract returns an different result from default Extract?

The difference is that Extract is a distributive conditional type, while your CustomExtract is not. In order for a conditional type of the form X extends Y ? A : B to be distributive, the checked type X must be a “naked type parameter”; that is, a type parameter like T in interface Foo<T> {…}, … Read more

TypeScript union of string and string literals

This is currently considered a design limitation (see microsoft/TypeScript#29729) and/or a missing feature (see microsoft/TypeScript#33471) of TypeScript. From the type system’s point of view, string is a supertype of any string literal like “blue” or “red”, and so a union string | “blue” | “red” is the same as string and the compiler aggressively reduces … Read more

Typescript: How can I make entries in an ES6 Map based on an object key/value type

Here’s the closest I can imagine getting, although I still don’t understand why we don’t just use plain objects to begin with: type ObjectToEntries<O extends object> = { [K in keyof O]: [K, O[K]] }[keyof O] interface ObjectMap<O extends object> { forEach(callbackfn: <K extends keyof O>( value: O[K], key: K, map: ObjectMap<O> ) => void, … Read more

TypeScript: void return type converted to any type?

This is by design (I’ll explain why it’s good design shortly). The spec says (in section 3.6.3, abridged for clarity): A type S is assignable to a type T, and T is assignable from S, if one of the following is true… S and T are object types and, for each member M in T, … Read more