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member-delimiter-style

danger
We strongly recommend you do not use this rule or any other formatting linter rules. Use a separate dedicated formatter instead. See What About Formatting? for more information.

Require a specific member delimiter style for interfaces and type literals.

🛠

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

TypeScript allows three delimiters between members in interfaces and type aliases:

interface Foo {
    // Semicolons (default, preferred in TypeScript):
    name: string;

    // Commas (JSON-like):
    name: string,

    // Line breaks (none):
    name: string
}

For code readability, it's generally best to use the same style consistently in your codebase.

This rule enforces keeping to one configurable code style. It can also standardize the presence (or absence) of a delimiter in the last member of a construct, as well as a separate delimiter syntax for single line declarations.

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/member-delimiter-style": "warn"
  }
};

Options​

This rule accepts an options object with the following properties:

interface Options {
  multiline?: {
    delimiter?: "none" | "semi" | "comma";
    requireLast?: boolean;
  };
  singleline?: {
    delimiter?: "semi" | "comma";
    requireLast?: boolean;
  };
  overrides?: {
    interface?: {
      multiline?: {
        delimiter?: "none" | "semi" | "comma";
        requireLast?: boolean;
      };
      singleline?: {
        delimiter?: "semi" | "comma";
        requireLast?: boolean;
      };
    };
    typeLiteral?: {
      multiline?: {
        delimiter?: "none" | "semi" | "comma";
        requireLast?: boolean;
      };
      singleline?: {
        delimiter?: "semi" | "comma";
        requireLast?: boolean;
      };
    };
  };
  multilineDetection?: "brackets" | "last-member";
}

const defaultOptions: Options = [
  {
    multiline: { delimiter: "semi", requireLast: true },
    singleline: { delimiter: "semi", requireLast: false },
    multilineDetection: "brackets",
  },
];

Default config:

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

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.

multilineDetection determines what counts as multiline

  • "brackets" (default) any newlines in the type or interface make it multiline.
  • "last-member" if the last member of the interface is on the same line as the last bracket, it is counted as a single line.

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

none 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 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; }

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.

Resources​