Disallows usage of void type outside of generic or return types (no-invalid-void-type)

Disallows usage of void type outside of return types or generic type arguments. If void is used as return type, it shouldn’t be a part of intersection/union type with most other types.

Rationale

The void type means “nothing” or that a function does not return any value, in contrast with implicit undefined type which means that a function returns a value undefined. So “nothing” cannot be mixed with any other types, other than never, which accepts all types. If you need this - use the undefined type instead.

Rule Details

This rule aims to ensure that the void type is only used in valid places.

Examples of code for this rule:

❌ Incorrect

type PossibleValues = string | number | void;
type MorePossibleValues = string | ((number & any) | (string | void));

function logSomething(thing: void) {}
function printArg<T = void>(arg: T) {}

logAndReturn<void>(undefined);

interface Interface {
  lambda: () => void;
  prop: void;
}

class MyClass {
  private readonly propName: void;
}

✅ Correct

type NoOp = () => void;

function noop(): void {}

let trulyUndefined = void 0;

async function promiseMeSomething(): Promise<void> {}

type stillVoid = void | never;

Options

interface Options {
  allowInGenericTypeArguments?: boolean | string[];
  allowAsThisParameter?: boolean;
}

const defaultOptions: Options = {
  allowInGenericTypeArguments: true,
  allowAsThisParameter: false,
};

allowInGenericTypeArguments

This option lets you control if void can be used as a valid value for generic type parameters.

Alternatively, you can provide an array of strings which whitelist which types may accept void as a generic type parameter.

Any types considered valid by this option will be considered valid as part of a union type with void.

This option is true by default.

The following patterns are considered warnings with { allowInGenericTypeArguments: false }:

logAndReturn<void>(undefined);

let voidPromise: Promise<void> = new Promise<void>(() => {});
let voidMap: Map<string, void> = new Map<string, void>();

The following patterns are considered warnings with { allowInGenericTypeArguments: ['Ex.Mx.Tx'] }:

logAndReturn<void>(undefined);

type NotAllowedVoid1 = Mx.Tx<void>;
type NotAllowedVoid2 = Tx<void>;
type NotAllowedVoid3 = Promise<void>;

The following patterns are not considered warnings with { allowInGenericTypeArguments: ['Ex.Mx.Tx'] }:

type AllowedVoid = Ex.Mx.Tx<void>;
type AllowedVoidUnion = void | Ex.Mx.Tx<void>;

allowAsThisParameter

This option allows specifying a this parameter of a function to be void when set to true. This pattern can be useful to explicitly label function types that do not use a this argument. See the TypeScript docs for more information.

This option is false by default.

The following patterns are considered warnings with { allowAsThisParameter: false } but valid with { allowAsThisParameter: true }:

function doThing(this: void) {}
class Example {
  static helper(this: void) {}
  callback(this: void) {}
}

When Not To Use It

If you don't care about if void is used with other types, or in invalid places, then you don't need this rule.

Related To

• TSLint: invalid-void

Attributes

• ✅ Recommended
• 🔧 Fixable
• 💭 Requires type information