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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​

// 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;
  }
}
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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 expression. */
    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​

When this option is true, the rule ignores arguments that are explicitly typed as any.

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

When this option is true, the rule ignores return type annotations on body-less arrow functions that return an as const type assertion.

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

When this option is true, the rule ignores return type annotations on function, which is immediately returning another function expression.

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

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

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

When this option is true, the rule ignores type annotations on the variable of a function expression.

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

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

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

export const foo = bar => {};
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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​

Resources​