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
Add the following line before other references to JavaScript. This is a nice little hack. <script>var exports = {};</script>
UPDATE Feb 2019 In TypeScript 3.4, which should be released in March 2019 it will be possible to tell the compiler to infer the type of a tuple of literals as a tuple of literals, instead of as, say, string[], by using the as const syntax. This type of assertion causes the compiler to infer … Read more
! is non-null assertion operator (post-fix expression) – it just saying to type checker that you’re sure that a is not null or undefined. the operation a! produces a value of the type of a with null and undefined excluded Optional chaining finally made it to typescript (3.7) 🎉 The optional chaining operator ?. permits … Read more
First, consider the following notation from your original question: export const addTodo3 = (text: string) => <AddTodoAction>({ type: “ADD_TODO”, text }) Using this notation, you typecast the returned object to the type AddTodoAction. However, the function’s declared return type is still undefined (and the compiler will implicitly assume any as return type). Use the following … Read more
To create a new object within generic code, you need to refer to the type by its constructor function. So instead of writing this: function activatorNotWorking<T extends IActivatable>(type: T): T { return new T(); // compile error could not find symbol T } You need to write this: function activator<T extends IActivatable>(type: { new(): T … Read more
Complementing @fetzz great answer. SHORT ANSWER TLDR; There are two common causes for this kind of error message. You are doing the first one (see below). Along with the text, I explain in rich detail what this error message wants to convey. CAUSE 1: In typescript, a concrete instance is not allowed to be assigned … Read more
A few tips in order restart the terminal restart the machine reinstall nodejs + then run npm install typescript -g If it still doesn’t work run npm config get prefix to see where npm install -g is putting files (append bin to the output) and make sure that they are in the path (the node … Read more
With the help of the Exclude type which was added in TypeScript 2.8, a generalizable way to require at least one of a set of properties is provided is: type RequireAtLeastOne<T, Keys extends keyof T = keyof T> = Pick<T, Exclude<keyof T, Keys>> & { [K in Keys]-?: Required<Pick<T, K>> & Partial<Pick<T, Exclude<Keys, K>>> }[Keys] … Read more
After two years of TypeScript development, I’ve finally managed to solve this problem. Basically, TypeScript has two kind of module types declaration: “local” (normal modules) and ambient (global). The second kind allows to write global modules declaration that are merged with existing modules declaration. What are the differences between this files? d.ts files are treated … Read more