Flag to disallow assignment of `void` to `any`

[stevenwdv]

Suggestion

I think it would be useful to have an option to forbid the assignment of void values to variables/parameters of type any.

🔍 Search Terms

flag assign why is void assignable to any

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

My suggestion is to add a flag to disallow assignment of void values to variables/parameters of type any.
As I understand it, writing void as a return value means that the value will not be observed. Assigning the value to a variable of type any thus seems like an undesirable action, and I wonder why this is even allowed at all.

📃 Motivating Example

Simple example

Current behavior:

const x: () => void = () => 42; // So far so good
console.log(x()); // Prints 42; this shouldn't be allowed because `x` returns void

Desired behavior if suggestion is implemented and flag is active:

const x: () => void = () => 42;
console.log(x()); // Error: void is not assignable to any

More involved example

const thanks = document.createElement("span");
thanks.textContent = "Thanks for supporting us!";
thanks.hidden = true;
const showThanksMessage: () => void = () => thanks.hidden = false;

const support = document.createElement("a");
support.textContent = "Support us!";
support.href = "/donate";
support.target = "_blank";
support.onclick = showThanksMessage;

document.body.append(support, " ", thanks);

Notice that when you click the support link, the donation page does not actually open. This is because showThanksMessage, which has a void return type, secretly returns false. This is then assigned to the onclick callback, which is a function returning any. This is then interpreted by the browser as 'prevent the default action of opening the link' (nowadays most people would use Event#preventDefault() to do this).

With the proposed flag enabled, the assignment to onclick would be an error, as we would not be able to assign () => void to (ev: MouseEvent) => any because of the return types.

💻 Use Cases

This prevents people from accidentally using the values of void-returning functions, as demonstrated above.

[MartinJohns]

Related: #42709

[stevenwdv]

I'm closing this as it has no chance of being implemented if even #50752 is not feasible.