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.

.eslintrc.cjs

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

Try this rule in the playground ↗

Options

This rule accepts an options object with the following properties:

interface Options {
  arrayDestructuring?: boolean;
  arrowParameter?: boolean;
  memberVariableDeclaration?: boolean;
  objectDestructuring?: boolean;
  parameter?: boolean;
  propertyDeclaration?: boolean;
  variableDeclaration?: boolean;
  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.

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

❌ Incorrect

const [a] = [1];
const [b, c] = [1, 2];

✅ Correct

const [a]: number[] = [1];
const [b]: [number] = [2];
const [c, d]: [boolean, string] = [true, 'text'];

for (const [key, val] of new Map([['key', 1]])) {
}

arrowParameter

Whether to enforce type annotations for parameters of arrow functions.

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

❌ Incorrect

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

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

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

✅ Correct

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

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

const mapper = {
  map: (text: string) => text + '...',
};

memberVariableDeclaration

Whether to enforce type annotations on member variables of classes.

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

❌ Incorrect

class ContainsText {
  delayedText;
  immediateTextImplicit = 'text';
}

✅ Correct

class ContainsText {
  delayedText: string;
  immediateTextImplicit: string = 'text';
}

objectDestructuring

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

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

❌ Incorrect

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

✅ Correct

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

for (const { key, val } of [{ key: 'key', val: 1 }]) {
}

parameter

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

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

❌ Incorrect

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;
  }
}

✅ Correct

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

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

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

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

propertyDeclaration

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

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

❌ Incorrect

type Members = {
  member;
  otherMember;
};

✅ Correct

type Members = {
  member: boolean;
  otherMember: string;
};

variableDeclaration

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

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

❌ Incorrect

const text = 'text';
let initialText = 'text';
let delayedText;

✅ Correct

const text: string = 'text';
let initialText: string = 'text';
let delayedText: string;

variableDeclarationIgnoreFunction

Ignore variable declarations for non-arrow and arrow functions.

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

❌ Incorrect

const text = 'text';

✅ Correct

const a = (): void => {};
const b = function (): void => {};
const c: () => void = (): void => {};

class Foo {
  a = (): void => {};
  b = function (): void => {};
  c = () => void = (): void => {};
}

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

• TypeScript Type System
• Type Inference

Resources

• Rule source
• Test source