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ban-types

Disallows certain types.

Some builtin types have aliases, some types are considered dangerous or harmful. It's often a good idea to ban certain types to help with consistency and safety.

Attributes

  • Included in configs
    • ✅ Recommended
    • 🔒 Strict
  • Fixable
    • 🔧 Automated Fixer
    • 🛠 Suggestion Fixer
  • 💭 Requires type information

Rule Details​

This rule bans specific types and can suggest alternatives. Note that it does not ban the corresponding runtime objects from being used.

Options​

type Options = {
  types?: {
    [typeName: string]:
      | false
      | string
      | {
          message: string;
          fixWith?: string;
        };
  };
  extendDefaults?: boolean;
};

The rule accepts a single object as options.

types​

An object whose keys are the types you want to ban, and the values are error messages.

The type can either be a type name literal (Foo), a type name with generic parameter instantiation(s) (Foo<Bar>), the empty object literal ({}), or the empty tuple type ([]).

The values can be:

  • A string, which is the error message to be reported; or
  • false to specifically un-ban this type (useful when you are using extendDefaults); or
  • An object with the following properties:
    • message: string - the message to display when the type is matched.
    • fixWith?: string - a string to replace the banned type with when the fixer is run. If this is omitted, no fix will be done.

extendDefaults​

If you're specifying custom types, you can set this to true to extend the default types configuration. This is a convenience option to save you copying across the defaults when adding another type.

If this is false, the rule will only use the types defined in your configuration.

Example configuration:

{
  "@typescript-eslint/ban-types": [
    "error",
    {
      "types": {
        // add a custom message to help explain why not to use it
        "Foo": "Don't use Foo because it is unsafe",

        // add a custom message, AND tell the plugin how to fix it
        "OldAPI": {
          "message": "Use NewAPI instead",
          "fixWith": "NewAPI"
        },

        // un-ban a type that's banned by default
        "{}": false
      },
      "extendDefaults": true
    }
  ]
}

Default Options​

The default options provide a set of "best practices", intended to provide safety and standardization in your codebase:

  • Don't use the upper-case primitive types, you should use the lower-case types for consistency.
  • Avoid the Function type, as it provides little safety for the following reasons:
    • It provides no type safety when calling the value, which means it's easy to provide the wrong arguments.
    • It accepts class declarations, which will fail when called, as they are called without the new keyword.
  • Avoid the Object and {} types, as they mean "any non-nullish value".
important

The default options suggest using Record<string, unknown>; this was a stylistic decision, as the built-in Record type is considered to look cleaner.

Default Options
const defaultTypes = {
  String: {
    message: 'Use string instead',
    fixWith: 'string',
  },
  Boolean: {
    message: 'Use boolean instead',
    fixWith: 'boolean',
  },
  Number: {
    message: 'Use number instead',
    fixWith: 'number',
  },
  Symbol: {
    message: 'Use symbol instead',
    fixWith: 'symbol',
  },
  BigInt: {
    message: 'Use bigint instead',
    fixWith: 'bigint',
  },

  Function: {
    message: [
      'The `Function` type accepts any function-like value.',
      'It provides no type safety when calling the function, which can be a common source of bugs.',
      'It also accepts things like class declarations, which will throw at runtime as they will not be called with `new`.',
      'If you are expecting the function to accept certain arguments, you should explicitly define the function shape.',
    ].join('\n'),
  },

  // object typing
  Object: {
    message: [
      'The `Object` type actually means "any non-nullish value", so it is marginally better than `unknown`.',
      '- If you want a type meaning "any object", you probably want `Record<string, unknown>` instead.',
      '- If you want a type meaning "any value", you probably want `unknown` instead.',
    ].join('\n'),
  },
  '{}': {
    message: [
      '`{}` actually means "any non-nullish value".',
      '- If you want a type meaning "any object", you probably want `Record<string, unknown>` instead.',
      '- If you want a type meaning "any value", you probably want `unknown` instead.',
    ].join('\n'),
  },
};

Examples​

Examples of code with the default options:

// use lower-case primitives for consistency
const str: String = 'foo';
const bool: Boolean = true;
const num: Number = 1;
const symb: Symbol = Symbol('foo');
const bigInt: BigInt = 1n;

// use a proper function type
const func: Function = () => 1;

// use safer object types
const capitalObj1: Object = 1;
const capitalObj2: Object = { a: 'string' };

const curly1: {} = 1;
const curly2: {} = { a: 'string' };