I believe this is because the type for [] is inferred to be never[], which is the type for an array that MUST be empty. You can use a type cast to address this: [‘a’, ‘b’, ‘c’].reduce((accumulator, value) => accumulator.concat(value), [] as string[]); Normally this wouldn’t be much of a problem since TypeScript does a … Read more
The compiler will widen string literal type to string, unless some specific conditions are met as explained in github issues and PR, or const assertion is used for literal value. Const assertions appeared in TypeScript 3.4: const KeyToVal = { MyKey1: ‘myValue1’, MyKey2: ‘myValue2’, } as const; type Keys = keyof typeof KeyToVal; type Values … Read more
Your object looks like a dictionary of Object arrays interface Dic { [key: string]: Object[] } The typescript literature often refers to this pattern as “the description of an indexable object” with a general form interface Dic { [key: string|number]: object_type } or type Dic = { [key: string|number]: object_type }
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];
This behaviour is controlled by the following option: Settings/Preferences Editor | Code Style | TypeScript “Punctuation” tab | Generated code -> Quote marks As of 2017.1 version it’s on new Punctuation tab and options are named a bit differently:
So isn’t this just: type HEX_CODE = `#${string}`; typescript playground
The point of the any type is to opt out of type checking for some parts of your code. It is intentionally unsound; all types are assignable to any and any is assignable to all types (except for never). This is both useful and dangerous. Useful because there are times where it tedious, difficult, or … Read more
If I understand correctly, you want OneKey<“a” | “b”> to be something like {a: any, b?: never} | {a?: never, b: any}. Meaning that it either has an a key or a b key but not both. So you want the type to be some sort of union to represent the either-or part of it. … Read more
I don’t completely agree with the premise of the default configuration of typescript-eslint’s ban-types rule which says that Avoid the object type, as it is currently hard to use due to not being able to assert that keys exist. See microsoft/TypeScript#21732. As the person who filed the linked issue, I do understand that it is … Read more
If you define the array first, here’s a way to get the Fruit type const arrayWithAllFruitsAsvalues = [‘apple’, ‘peach’] as const; type Fruit = typeof arrayWithAllFruitsAsvalues[number]; // “apple” | “peach” const objectWithAllFruitsAsKeys: { [ key in Fruit ]: any } = { apple: ”, peach: ” }