The difference is what you can do with favoriteColors after you define it. With satisfies, the actual type of the assigned object literal is used as the type of favoriteColors, the information about what keys and values it contains is preserved (the type of the object literal is just checked against the type it’s supposed … Read more
$event is the event itself. The event binding (someevent) allows to bind to DOM events and to EventEmitter events. The syntax is exactly the same. For DOM events $event is the MouseEvent, KeyboardEvent, or the event value of the type of whatever event you listen to. For EventEmitter events the emitted value is available as … Read more
This is currently a limitation of TypeScript as described in microsoft/TypeScript#10530. You’re expecting that by checking feeObj[property] !== undefined that TypeScript will narrow the type of feeObj[property] from number | undefined to undefined, but unfortunately that isn’t happening. TypeScript does normally allow such equality narrowing on property accesses, as shown here: function works(feeObj: FeeObjType2) { … Read more
Yeah, there’s no convenient and type-safe solution for you here. I’ve opened an issue at microsoft/TypeScript#30581 about this but I don’t expect it to be addressed. I see two main ways forward. One is to just use a type assertion, since you legitimately know more than the compiler does here. It could be like this: … Read more
The use case you’re mentioning, coming up with a tuple type for Object.keys(), is fraught with peril and I recommend against using it. The first issue is that types in TypeScript are not “exact”. That is, just because I have a value of type Person, it does not mean that the value contains only the … Read more
As written, type OptionalArgBroken<Arg> = Arg extends undefined ? () => void : (arg: Arg) => void; is a distributive conditional type because the type being checked, Arg, is a naked generic type parameter. “Distributive” means that if the Arg passed in is a union, then the type will be evaluated for each member of … Read more
Naming in computer science is notoriously difficult. In TypeScript, we generally use the same name to refer to the constructor of a class (which is a value that exists at runtime), and as the type of the instances of the class (which only exists at design time and is not emitted to JavaScript). So in … Read more
(update 2021-06-10, this is unchanged by the TS4.3 update for union enums) This is (maybe surprisingly) intended behavior. Numeric enums in TypeScript are sometimes used for bitwise operations, where the listed values are treated as flags. And, as stated by @RyanCavanaugh in a comment on a reported issue about this: We don’t distinguish between flag … Read more
In fact, by default Angular2 detects changes when object reference is updated not its content. But this default behavior can be changed by using the DoCheck interface. In your case (detecting that a property was updated into the myObject object, you could use the following approach: @Component({ selector: ‘my-component’, (…) }) export class MyComponent implements … Read more
This is now possible to a limited extent with TypeScript 4.6’s Control Flow Analysis for Dependent Parameters. TypeScript can narrow down the types of arguments if the narrowing is done with ===. (A typeof check doesn’t appear to work, at least not yet.) To tweak the example in the question, if the first argument, when … Read more