S and T are object types and, for each member M in T, one of the following is true: M is a call, construct or index signature and S contains a call, construct or index signature N where. Return type void. We make it clear which is the return type by using an arrow (=>) between the parameters and the return type. Solution When a Promise resolves with nothing, then you need to define this return type ( void ) in recent versions of TypeScript. It means the absence of any type. Require explicit return types on functions and class methods (explicit-function-return-type) Explicit types for function return values makes it clear to any calling code what type is returned. void in JavaScript # void in JavaScript is an operator which evaluates the expression next to it. It is possible to specify void as a type parameter for generic function in TypeScript. The second part is the return type. Did you forget to include ‘void’ in your type argument to ‘Promise’? No matter which expression is evaluated, void always returns undefined. Important to note that void and any types overlap a bit, which may be weird at first. However when void is a type of an argument it's not that clear what value can be used for it. Then, I figured out that TypeScript infers the type of the callback's parameter e to be an intersection(&) of MyMouseEvent and MyKeyboardEvent: e: MyEventObjects[K] >>>> e: MyMouseEvent & MyKeyboardEvent Don’t use any as a type unless you are in the process of void exists in both JavaScript as an operator and in TypeScript as a primitive type. Have your JS expressiveness and type it too. Generics. Don’t ever have a generic type which doesn’t use its type parameter. Nullable types. We mentioned these briefly in the Basic Types section. Instead of Object, use the non-primitive object type (added in TypeScript 2.2). In Typescript (using in an Angular project) for a method that returns nothing (void), which of the below is best practice? Allow me to quickly answer to "normal" use case of "How to define function overload types with TypeScript" with an example: I want a function that accepts a callback or returns a promise if none is provided: Then we get Type ‘1’ is not assignable to type ‘object’ and the code can’t be compiled with the TypeScript compiler and be run, since 1 is a primitive value. the result type of M is Void, or the result type of N is assignable to that of M. In this case, we're testing if => string is assignable to => void. By default, the type checker considers null and undefined assignable to anything. As mentioned before, this is a required part of the function type, so if the function doesn’t return a value, you would use void instead of leaving it off. And it works fine for return values of functions. And in both worlds void works a little bit different than most people are used to. See more details in TypeScript FAQ page. any. TypeScript has two special types, null and undefined, that have the values null and undefined respectively. But, this didn't happen, because TypeScript didn't recognise the relationship between eventName: K and callback: (e: MyEventObjects[K]) => void. The void type is pretty much the opposite of the any type. onSelect(someNumber: number): void { } OR. Contextual typing with a return type of void does not force functions to not return something. The void return type for functions can produce some unusual, but expected behavior. onSelect(someNumber: number) { } I've seen it both ways in different examples and wasn't sure if it is better to add the return type as void … Is there a way to specify a parameter for an argument of the void type?

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