member-delimiter-style.md

Require a specific member delimiter style for interfaces and type literals (member-delimiter-style)

Enforces a consistent member delimiter style in interfaces and type literals. There are three member delimiter styles primarily used in TypeScript:

• Semicolon style (default, preferred in TypeScript).

interface Foo {
    name: string;
    greet(): void;
}

type Bar = {
    name: string;
    greet(): void;
}

• Comma style (JSON style).

interface Foo {
    name: string,
    greet(): void,
}

type Bar = {
    name: string,
    greet(): void,
}

• Linebreak (none) style.

interface Foo {
    name: string
    greet(): void
}

type Bar = {
    name: string
    greet(): void
}

The rule also enforces the presence (or absence) of the delimiter in the last member of the interface and/or type literal. Finally, this rule can enforce separate delimiter syntax for single line declarations.

Rule Details

This rule aims to standardise the way interface and type literal members are delimited.

Options

interface BaseConfig {
    multiline?: {
        delimiter?: "none" | "semi" | "comma";
        requireLast?: boolean;
    };
    singleline?: {
        delimiter?: "semi" | "comma";
        requireLast?: boolean;
    };
}
type Config = BaseConfig & {
    overrides?: {
        interface?: BaseConfig;
        typeLiteral?: BaseConfig;
    };
};

Default config:

{
    "multiline": {
        "delimiter": "semi",
        "requireLast": true
    },
    "singleline": {
        "delimiter": "semi",
        "requireLast": false
    }
}

multiline config only applies to multiline interface/type definitions. singleline config only applies to single line interface/type definitions. The two configs are entirely separate, and do not effect one another.

delimiter

Accepts three values (or two for singleline):

• comma - each member should be delimited with a comma (,).
• semi - each member should be delimited with a semicolon (;).
• none - each member should be delimited with nothing.
  • NOTE - this is not an option for singleline because having no delimiter between members on a single line is a syntax error in TS.

requireLast

Determines whether or not the last member in the interface/type should have a delimiter:

• true - the last member must have a delimiter.
• false - the last member must not have a delimiter.

overrides

Allows you to specify options specifically for either interfaces or type definitions / inline types.

For example, to require commas for types, and semicolons for multiline interfaces:

{
    "multiline": {
        "delimiter": "comma",
        "requireLast": true
    },
    "singleline": {
        "delimiter": "comma",
        "requireLast": true
    },
    "overrides": {
        "interface": {
            "multiline": {
                "delimiter": "semi",
                "requireLast": true
            }
        }
    }
}

Examples

Examples of incorrect code for this rule with the default config:

// missing semicolon delimiter
interface Foo {
    name: string
    greet(): string
}

// using incorrect delimiter
interface Bar {
    name: string,
    greet(): string,
}

// missing last member delimiter
interface Baz {
    name: string;
    greet(): string
}

// incorrect delimiter
type FooBar = { name: string, greet(): string }

// last member should not have delimiter
type FooBar = { name: string; greet(): string; }

Examples of correct code for this rule with the default config:

interface Foo {
    name: string;
    greet(): string;
}

interface Foo { name: string }

type Bar = {
    name: string;
    greet(): string;
}

type Bar = { name: string }

type FooBar = { name: string; greet(): string }

When Not To Use It

If you don't care about enforcing a consistent member delimiter in interfaces and type literals, then you will not need this rule.