---
description: "Require explicit return and argument types on exported functions' and classes' public class methods."
---

import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';

> 🛑 This file is source code, not the primary documentation location! 🛑
>
> See **https://typescript-eslint.io/rules/explicit-module-boundary-types** for documentation.

Explicit types for function return values and arguments makes it clear to any calling code what is the module boundary's input and output.
Adding explicit type annotations for those types can help improve code readability.
It can also improve TypeScript type checking performance on larger codebases.

## Examples

<Tabs>
<TabItem value="❌ Incorrect">

```ts
// Should indicate that no value is returned (void)
export function test() {
  return;
}

// Should indicate that a string is returned
export var arrowFn = () => 'test';

// All arguments should be typed
export var arrowFn = (arg): string => `test ${arg}`;
export var arrowFn = (arg: any): string => `test ${arg}`;

export class Test {
  // Should indicate that no value is returned (void)
  method() {
    return;
  }
}
```

</TabItem>
<TabItem value="✅ Correct">

```ts
// A function with no return value (void)
export function test(): void {
  return;
}

// A return value of type string
export var arrowFn = (): string => 'test';

// All arguments should be typed
export var arrowFn = (arg: string): string => `test ${arg}`;
export var arrowFn = (arg: unknown): string => `test ${arg}`;

export class Test {
  // A class method with no return value (void)
  method(): void {
    return;
  }
}

// The function does not apply because it is not an exported function.
function test() {
  return;
}
```

</TabItem>
</Tabs>

## Options

### Configuring in a mixed JS/TS codebase

If you are working on a codebase within which you lint non-TypeScript code (i.e. `.js`/`.mjs`/`.cjs`/`.jsx`), you should ensure that you should use [ESLint `overrides`](https://eslint.org/docs/user-guide/configuring#disabling-rules-only-for-a-group-of-files) to only enable the rule on `.ts`/`.mts`/`.cts`/`.tsx` files. If you don't, then you will get unfixable lint errors reported within `.js`/`.mjs`/`.cjs`/`.jsx` files.

```jsonc
{
  "rules": {
    // disable the rule for all files
    "@typescript-eslint/explicit-module-boundary-types": "off",
  },
  "overrides": [
    {
      // enable the rule specifically for TypeScript files
      "files": ["*.ts", "*.mts", "*.cts", "*.tsx"],
      "rules": {
        "@typescript-eslint/explicit-module-boundary-types": "error",
      },
    },
  ],
}
```

### `allowArgumentsExplicitlyTypedAsAny`

{/* insert option description */}

Examples of code for this rule with `{ allowArgumentsExplicitlyTypedAsAny: false }`:

<Tabs>
<TabItem value="❌ Incorrect">

```ts option='{ "allowArgumentsExplicitlyTypedAsAny": false }'
export const func = (value: any): number => value + 1;
```

</TabItem>
<TabItem value="✅ Correct">

```ts option='{ "allowArgumentsExplicitlyTypedAsAny": true }'
export const func = (value: any): number => value + 1;
```

</TabItem>
</Tabs>

### `allowDirectConstAssertionInArrowFunctions`

{/* insert option description */}

Examples of code for this rule with `{ allowDirectConstAssertionInArrowFunctions: false }`:

<Tabs>
<TabItem value="❌ Incorrect">

```ts option='{ "allowDirectConstAssertionInArrowFunctions": false }'
export const func = (value: number) => ({ type: 'X', value });
export const foo = () => ({
  bar: true,
});
export const bar = () => 1;
```

</TabItem>
<TabItem value="✅ Correct">

```ts option='{ "allowDirectConstAssertionInArrowFunctions": true }'
export const func = (value: number) => ({ type: 'X', value }) as const;
export const foo = () =>
  ({
    bar: true,
  }) as const;
export const bar = () => 1 as const;
```

</TabItem>
</Tabs>

### `allowedNames`

{/* insert option description */}

You may pass function/method names you would like this rule to ignore, like so:

```json
{
  "@typescript-eslint/explicit-module-boundary-types": [
    "error",
    {
      "allowedNames": ["ignoredFunctionName", "ignoredMethodName"]
    }
  ]
}
```

### `allowHigherOrderFunctions`

{/* insert option description */}

Examples of code for this rule with `{ allowHigherOrderFunctions: false }`:

<Tabs>
<TabItem value="❌ Incorrect">

```ts option='{ "allowHigherOrderFunctions": false }'
export const arrowFn = () => () => {};

export function fn() {
  return function () {};
}

export function foo(outer: string) {
  return function (inner: string) {};
}
```

</TabItem>
<TabItem value="✅ Correct">

```ts option='{ "allowHigherOrderFunctions": true }'
export const arrowFn = () => (): void => {};

export function fn() {
  return function (): void {};
}

export function foo(outer: string) {
  return function (inner: string): void {};
}
```

</TabItem>
</Tabs>

### `allowTypedFunctionExpressions`

{/* insert option description */}

Examples of code for this rule with `{ allowTypedFunctionExpressions: false }`:

<Tabs>
<TabItem value="❌ Incorrect">

```ts option='{ "allowTypedFunctionExpressions": false }'
export let arrowFn = () => 'test';

export let funcExpr = function () {
  return 'test';
};

export let objectProp = {
  foo: () => 1,
};

export const foo = bar => {};
```

</TabItem>
<TabItem value="✅ Correct">

```ts option='{ "allowTypedFunctionExpressions": true }'
type FuncType = () => string;

export let arrowFn: FuncType = () => 'test';

export let funcExpr: FuncType = function () {
  return 'test';
};

export let asTyped = (() => '') as () => string;
export let castTyped = <() => string>(() => '');

interface ObjectType {
  foo(): number;
}
export let objectProp: ObjectType = {
  foo: () => 1,
};
export let objectPropAs = {
  foo: () => 1,
} as ObjectType;
export let objectPropCast = <ObjectType>{
  foo: () => 1,
};

type FooType = (bar: string) => void;
export const foo: FooType = bar => {};
```

</TabItem>
</Tabs>

## When Not To Use It

If your project is not used by downstream consumers that are sensitive to API types, you can disable this rule.

## Further Reading

- TypeScript [Functions](https://www.typescriptlang.org/docs/handbook/functions.html#function-types)

## Related To

- [explicit-function-return-type](./explicit-function-return-type.mdx)
