explicit-module-boundary-types

Require explicit return and argument types on exported functions' and classes' public class methods.

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.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/explicit-module-boundary-types": "error"
  }
};

Try this rule in the playground ↗

Examples

❌ Incorrect

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

// Should indicate that a number is returned
export default function () {
  return 1;
}

// 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;
  }
}

✅ Correct

// Function is not exported
function test() {
  return;
}

// A return value of type number
export var fn = function (): number {
  return 1;
};

// 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}`;

// Class is not exported
class Test {
  method() {
    return;
  }
}

Options

This rule accepts the following options

type Options = [
  {
    /** Whether to ignore arguments that are explicitly typed as `any`. */
    allowArgumentsExplicitlyTypedAsAny?: boolean;
    /**
     * Whether to ignore return type annotations on body-less arrow functions that return an `as const` type assertion.
     * You must still type the parameters of the function.
     */
    allowDirectConstAssertionInArrowFunctions?: boolean;
    /**
     * Whether to ignore return type annotations on functions immediately returning another function expression.
     * You must still type the parameters of the function.
     */
    allowHigherOrderFunctions?: boolean;
    /** Whether to ignore type annotations on the variable of a function expresion. */
    allowTypedFunctionExpressions?: boolean;
    /** An array of function/method names that will not have their arguments or return values checked. */
    allowedNames?: string[];
  },
];

const defaultOptions: Options = [
  {
    allowArgumentsExplicitlyTypedAsAny: false,
    allowDirectConstAssertionInArrowFunctions: true,
    allowedNames: [],
    allowHigherOrderFunctions: true,
    allowTypedFunctionExpressions: true,
  },
];

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 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.

{
  "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

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

❌ Incorrect

export const func = (value: any): number => value + 1;

✅ Correct

export const func = (value: number): number => value + 1;

allowDirectConstAssertionInArrowFunctions

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

❌ Incorrect

export const func = (value: number) => ({ type: 'X', value });
export const foo = () => ({
  bar: true,
});
export const bar = () => 1;

✅ Correct

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

allowedNames

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

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

allowHigherOrderFunctions

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

❌ Incorrect

export const arrowFn = () => () => {};

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

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

✅ Correct

export const arrowFn = () => (): void => {};

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

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

allowTypedFunctionExpressions

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

❌ Incorrect

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

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

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

export const foo = bar => {};

✅ Correct

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 => {};

When Not To Use It

If you wish to make sure all functions have explicit return types, as opposed to only the module boundaries, you can use explicit-function-return-type

Further Reading

• TypeScript Functions

Resources

• Rule source
• Test source