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typedef

Require type annotations in certain places.

TypeScript cannot always infer types for all places in code. Some locations require type annotations for their types to be inferred.

This rule can enforce type annotations in locations regardless of whether they're required. This is typically used to maintain consistency for element types that sometimes require them.

class ContainsText {
  // There must be a type annotation here to infer the type
  delayedText: string;

  // `typedef` requires a type annotation here to maintain consistency
  immediateTextExplicit: string = 'text';

  // This is still a string type because of its initial value
  immediateTextImplicit = 'text';
}

To enforce type definitions existing on call signatures, use explicit-function-return-type, or explicit-module-boundary-types.

caution

Requiring type annotations unnecessarily can be cumbersome to maintain and generally reduces code readability. TypeScript is often better at inferring types than easily written type annotations would allow.

Instead of enabling typedef, it is generally recommended to use the --noImplicitAny and --strictPropertyInitialization compiler options to enforce type annotations only when useful.

eslint.config.mjs
export default tseslint.config({
  rules: {
    "@typescript-eslint/typedef": "error"
  }
});

Try this rule in the playground ↗

Options​

This rule accepts the following options:

type Options = [
  {
    /** Whether to enforce type annotations on variables declared using array destructuring. */
    arrayDestructuring?: boolean;
    /** Whether to enforce type annotations for parameters of arrow functions. */
    arrowParameter?: boolean;
    /** Whether to enforce type annotations on member variables of classes. */
    memberVariableDeclaration?: boolean;
    /** Whether to enforce type annotations on variables declared using object destructuring. */
    objectDestructuring?: boolean;
    /** Whether to enforce type annotations for parameters of functions and methods. */
    parameter?: boolean;
    /** Whether to enforce type annotations for properties of interfaces and types. */
    propertyDeclaration?: boolean;
    /** Whether to enforce type annotations for variable declarations, excluding array and object destructuring. */
    variableDeclaration?: boolean;
    /** Whether to ignore variable declarations for non-arrow and arrow functions. */
    variableDeclarationIgnoreFunction?: boolean;
  },
];

const defaultOptions: Options = [
  {
    arrayDestructuring: false,
    arrowParameter: false,
    memberVariableDeclaration: false,
    objectDestructuring: false,
    parameter: false,
    propertyDeclaration: false,
    variableDeclaration: false,
    variableDeclarationIgnoreFunction: false,
  },
];

For example, with the following configuration:

{
  "rules": {
    "@typescript-eslint/typedef": [
      "error",
      {
        "arrowParameter": true,
        "variableDeclaration": true
      }
    ]
  }
}
  • Type annotations on arrow function parameters are required
  • Type annotations on variables are required

arrayDestructuring​

Whether to enforce type annotations on variables declared using array destructuring. Default: false.

Examples of code with { "arrayDestructuring": true }:

const [a] = [1];
const [b, c] = [1, 2];
Open in Playground

arrowParameter​

Whether to enforce type annotations for parameters of arrow functions. Default: false.

Examples of code with { "arrowParameter": true }:

const logsSize = size => console.log(size);

['hello', 'world'].map(text => text.length);

const mapper = {
  map: text => text + '...',
};
Open in Playground

memberVariableDeclaration​

Whether to enforce type annotations on member variables of classes. Default: false.

Examples of code with { "memberVariableDeclaration": true }:

class ContainsText {
  delayedText;
  immediateTextImplicit = 'text';
}
Open in Playground

objectDestructuring​

Whether to enforce type annotations on variables declared using object destructuring. Default: false.

Examples of code with { "objectDestructuring": true }:

const { length } = 'text';
const [b, c] = Math.random() ? [1, 2] : [3, 4];
Open in Playground

parameter​

Whether to enforce type annotations for parameters of functions and methods. Default: false.

Examples of code with { "parameter": true }:

function logsSize(size): void {
  console.log(size);
}

const doublesSize = function (size): number {
  return size * 2;
};

const divider = {
  curriesSize(size): number {
    return size;
  },
  dividesSize: function (size): number {
    return size / 2;
  },
};

class Logger {
  log(text): boolean {
    console.log('>', text);
    return true;
  }
}
Open in Playground

propertyDeclaration​

Whether to enforce type annotations for properties of interfaces and types. Default: false.

Examples of code with { "propertyDeclaration": true }:

type Members = {
  member;
  otherMember;
};
Open in Playground

variableDeclaration​

Whether to enforce type annotations for variable declarations, excluding array and object destructuring. Default: false.

Examples of code with { "variableDeclaration": true }:

const text = 'text';
let initialText = 'text';
let delayedText;
Open in Playground

variableDeclarationIgnoreFunction​

Whether to ignore variable declarations for non-arrow and arrow functions. Default: false.

Examples of code with { "variableDeclaration": true, "variableDeclarationIgnoreFunction": true }:

const text = 'text';
Open in Playground

When Not To Use It​

If you are using stricter TypeScript compiler options, particularly --noImplicitAny and/or --strictPropertyInitialization, you likely don't need this rule.

In general, if you do not consider the cost of writing unnecessary type annotations reasonable, then do not use this rule.

Further Reading​

Resources​