Type ‘void’ is not assignable to type ‘((event: MouseEvent) => void) | undefined’

Type ‘void’ is not assignable to type ‘((event: MouseEvent) => void) | undefined’

When working with JavaScript, you may come across an error message similar to the one mentioned in the title: “Type ‘void’ is not assignable to type ‘((event: MouseEvent) => void) | undefined’”. This error usually occurs when you are trying to assign a function that returns ‘void’ to a variable or property that expects a specific type of function.
To better understand this error and how to resolve it, let’s break it down:

• Type ‘void’: ‘void’ is a type in JavaScript that represents the absence of a value. It is commonly used as the return type for functions that do not return anything.
• ‘((event: MouseEvent) => void) | undefined’: This is the expected type for the variable or property. It is a union type that can either be a function that takes a ‘MouseEvent’ of type ‘HTMLInputElement’ as an argument and returns ‘void’, or it can be ‘undefined’.

Now that we understand the error message, let’s explore some possible solutions:

Solution 1: Specify the correct function signature

The error message suggests that the expected type is a function that takes a ‘MouseEvent’ of type ‘HTMLInputElement’ as an argument and returns ‘void’. To resolve the error, make sure that the assigned function matches this signature:

const handleClick = (event: MouseEvent): void => {
    // Your code here
};
    

By specifying the correct function signature, the error should be resolved.

Solution 2: Use a type assertion

If you are certain that the assigned function is compatible with the expected type, you can use a type assertion to override the type checking:

const handleClick = (event: MouseEvent): void => {
    // Your code here
};

const onClick: ((event: MouseEvent) => void) | undefined = handleClick as ((event: MouseEvent) => void) | undefined;
    

By using a type assertion, you are essentially telling the compiler that you know the assigned function is of the correct type, even though it may not match the expected type exactly.
These are two possible solutions to resolve the error message “Type ‘void’ is not assignable to type ‘((event: MouseEvent) => void) | undefined’”. Choose the solution that best fits your use case and implement it accordingly.