The error is indeed fixed by inserting this with a type annotation as the first callback parameter. My attempt to do that was botched by simultaneously changing the callback into an arrow-function: foo.on(‘error’, (this: Foo, err: any) => { // DON’T DO THIS It should’ve been this: foo.on(‘error’, function(this: Foo, err: any) { or this: … Read more
if you write Component as React.FC, and using useState(), just write like this would be helpful: const [arr, setArr] = useState<any[]>([])
TypeScript uses ‘<>’ to surround casts, so the above becomes: var script = <HTMLScriptElement>document.getElementsByName(“script”)[0]; However, unfortunately you cannot do: var script = (<HTMLScriptElement[]>document.getElementsByName(id))[0]; You get the error Cannot convert ‘NodeList’ to ‘HTMLScriptElement[]’ But you can do : (<HTMLScriptElement[]><any>document.getElementsByName(id))[0];
found what I was looking for: Declare vs. var var creates a new variable. declare is used to tell TypeScript that the variable has been created elsewhere. If you use declare, nothing is added to the JavaScript that is generated – it is simply a hint to the compiler. For example, if you use an … Read more
For : abc:number|string; Use the JavaScript operator typeof: if (typeof abc === “number”) { // do something } TypeScript understands typeof 🌹 This is called a typeguard. More For classes you would use instanceof e.g. class Foo {} class Bar {} // Later if (fooOrBar instanceof Foo){ // TypeScript now knows that `fooOrBar` is `Foo` … Read more
You could use Map.prototype.forEach((value, key, map) => void, thisArg?) : void instead Use it like this: myMap.forEach((value: boolean, key: string) => { console.log(key, value); });
Typescript treats files without import/exports as legacy script files. As such files are not modules and any definitions they have get merged in the global namespace. isolatedModules forbids such files. Adding any import or export to a file makes it a module and the error disappears. Also export {} is a handy way to make … Read more
The name property is marked as protected. This was added in TypeScript 1.3 and is now firmly established. The makeSound method is marked as abstract, as is the class. You cannot directly instantiate an Animal now, because it is abstract. This is part of TypeScript 1.6, which is now officially live. abstract class Animal { … Read more
To understand the keyof typeof usage in TypeScript, first you need to understand what are literal types and union of literal types. So, I’ll explain these concepts first and then explain keyof and typeof individually in detail. After that, I’ll come back to enum to answer what is asked in the question. It’s a long … Read more