type-annotation-spacing.md

Require consistent spacing around type annotations (type-annotation-spacing)

Spacing around type annotations improves readability of the code. Although the most commonly used style guideline for type annotations in TypeScript prescribes adding a space after the colon, but not before it, it is subjective to the preferences of a project. For example:

// with space after, but not before (default if no option is specified)
let foo: string = "bar";

// with no spaces
let foo:string = "bar";

// with space before and after
let foo : string = "bar";

// with space before, but not after
let foo :string = "bar";

// with spaces before and after the fat arrow (default if no option is specified)
type Foo = (string: name) => string;

// with no spaces between the fat arrow
type Foo = (string: name)=>string;

// with space after, but not before the fat arrow
type Foo = (string: name)=> string;

// with space before, but not after the fat arrow
type Foo = (string: name) =>string;

Rule Details

This rule aims to enforce specific spacing patterns around type annotations and function types in type literals.

Options

This rule has an object option:

• "before": false, (default for colon) disallows spaces before the colon/arrow.
• "before": true, (default for arrow) requires a space before the colon/arrow.
• "after": true, (default) requires a space after the colon/arrow.
• "after": false, disallows spaces after the colon/arrow.
• "overrides", overrides the default options for type annotations with colon (e.g. const foo: string) and function types with arrow (e.g. type Foo = () => {}).

defaults

Examples of incorrect code for this rule with no options at all:

let foo:string = "bar";
let foo :string = "bar";
let foo : string = "bar";

function foo():string {}
function foo() :string {}
function foo() : string {}

class Foo {
    name:string;
}

class Foo {
    name :string;
}

class Foo {
    name : string;
}

type Foo = ()=> {};

Examples of correct code for this rule with no options at all:

let foo: string = "bar";

function foo(): string {}

class Foo {
    name: string;
}

type Foo = () => {};

after

Examples of incorrect code for this rule with { "before": false, "after": true }:

let foo:string = "bar";
let foo :string = "bar";
let foo : string = "bar";

function foo():string {}
function foo() :string {}
function foo() : string {}

class Foo {
    name:string;
}

class Foo {
    name :string;
}

class Foo {
    name : string;
}

type Foo = () => {};

Examples of correct code for this rule with { "before": false, "after": true }:

let foo: string = "bar";

function foo(): string {}

class Foo {
    name: string;
}

type Foo = ()=> {};

before

Examples of incorrect code for this rule with { "before": true, "after": true } options:

let foo: string = "bar";
let foo:string = "bar";
let foo :string = "bar";

function foo(): string {}
function foo():string {}
function foo() :string {}

class Foo {
    name: string;
}

class Foo {
    name:string;
}

class Foo {
    name :string;
}

Examples of correct code for this rule with { "before": true, "after": true } options:

let foo : string = "bar";

function foo() : string {}

class Foo {
    name : string;
}

overrides - colon

Examples of incorrect code for this rule with { "before": false, "after": false, overrides: { colon: { before: true, after: true }} } options:

let foo: string = "bar";
let foo:string = "bar";
let foo :string = "bar";

function foo(): string {}
function foo():string {}
function foo() :string {}

class Foo {
    name: string;
}

class Foo {
    name:string;
}

class Foo {
    name :string;
}

type Foo = {
    name: (name:string) => string;
}

Examples of correct code for this rule with { "before": true, "after": true, overrides: { colon: { before: true, after: true }} } options:

let foo : string = "bar";

function foo() : string {}

class Foo {
    name : string;
}

type Foo = {
    name: (name : string)=>string;
}

overrides - arrow

Examples of incorrect code for this rule with { "before": false, "after": false, overrides: { arrow: { before: true, after: true }} } options:

let foo: string = "bar";
let foo : string = "bar";
let foo :string = "bar";

function foo(): string {}
function foo():string {}
function foo() :string {}

class Foo {
    name: string;
}

class Foo {
    name : string;
}

class Foo {
    name :string;
}

type Foo = {
    name: (name : string)=>string;
}

type Foo = {
    name: (name : string) =>string;
}

type Foo = {
    name: (name : string)=> string;
}

Examples of correct code for this rule with { "before": false, "after": false, overrides: { arrow: { before: true, after: true }} } options:

let foo:string = "bar";

function foo():string {}

class Foo {
    name:string;
}

type Foo = {
    name: (name:string) => string;
}

When Not To Use It

If you don't want to enforce spacing for your type annotations, you can safely turn this rule off.

Further Reading

• TypeScript Type System
• Type Inference

Compatibility

• TSLint: typedef-whitespace