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, orexplicit-module-boundary-types.
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.
module.exports = {
"rules": {
"@typescript-eslint/typedef": "error"
}
};
Options
This rule accepts the following options
type 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
- ✅ Correct
const [a] = [1];
const [b, c] = [1, 2];
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
- ✅ Correct
const logsSize = size => console.log(size);
['hello', 'world'].map(text => text.length);
const mapper = {
map: text => text + '...',
};
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
- ✅ Correct
class ContainsText {
delayedText;
immediateTextImplicit = 'text';
}
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
- ✅ Correct
const { length } = 'text';
const [b, c] = Math.random() ? [1, 2] : [3, 4];
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
- ✅ Correct
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;
}
}
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
- ✅ Correct
type Members = {
member;
otherMember;
};
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
- ✅ Correct
const text = 'text';
let initialText = 'text';
let delayedText;
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
- ✅ Correct
const text = 'text';
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.