void type typescript

void in JavaScript # void in JavaScript is an operator which evaluates the expression next to it. 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. 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. And in both worlds void works a little bit different than most people are used to. No matter which expression is evaluated, void always returns undefined. The void type is pretty much the opposite of the any type. Generics. Nullable types. However when void is a type of an argument it's not that clear what value can be used for it. 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: We mentioned these briefly in the Basic Types section. It means the absence of any type. Don’t ever have a generic type which doesn’t use its type parameter. Did you forget to include ‘void’ in your type argument to ‘Promise’? Return type void. Instead of Object, use the non-primitive object type (added in TypeScript 2.2). 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. onSelect(someNumber: number): void { } OR. The second part is the return type. 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. And it works fine for return values of functions. void exists in both JavaScript as an operator and in TypeScript as a primitive type. See more details in TypeScript FAQ page. Don’t use any as a type unless you are in the process of Have your JS expressiveness and type it too. 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. 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 But, this didn't happen, because TypeScript didn't recognise the relationship between eventName: K and callback: (e: MyEventObjects[K]) => void. By default, the type checker considers null and undefined assignable to anything. It is possible to specify void as a type parameter for generic function in TypeScript. 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 … TypeScript has two special types, null and undefined, that have the values null and undefined respectively. Is there a way to specify a parameter for an argument of the void type? Important to note that void and any types overlap a bit, which may be weird at first. Contextual typing with a return type of void does not force functions to not return something. Solution When a Promise resolves with nothing, then you need to define this return type ( void ) in recent versions of TypeScript. In Typescript (using in an Angular project) for a method that returns nothing (void), which of the below is best practice? The void return type for functions can produce some unusual, but expected behavior. any. 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