Bans specific types from being used (ban-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.

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, with the following keys:

• 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>), or the empty object literal ({}).
  • The values can be a string, which is the error message to be reported, false to specifically disable this type or it can be 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
        "String": {
          "message": "Use string instead",
          "fixWith": "string"
        },

        "{}": {
          "message": "Use object instead",
          "fixWith": "object"
        }
      }
    }
  ]
}

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”.
  • This is a point of confusion for many developers, who think it means “any object type”.
• Avoid the object type, as it is currently hard to use due to not being able to assert that keys exist.
  • See microsoft/TypeScript#21732.

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

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',
  },

  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'),
  },
  object: {
    message: [
      'The `object` type is currently hard to use ([see this issue](https://github.com/microsoft/TypeScript/issues/21732)).',
      'Consider using `Record<string, unknown>` instead, as it allows you to more easily inspect and use the keys.',
    ].join('\n'),
  },
};

Examples

Examples of incorrect 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');

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

// use safer object types
const lowerObj: object = {};

const capitalObj1: Object = 1;
const capitalObj2: Object = { a: 'string' };

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

Examples of correct 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');

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

// use safer object types
const lowerObj: Record<string, unknown> = {};

const capitalObj1: number = 1;
const capitalObj2: { a: string } = { a: 'string' };

const curly1: number = 1;
const curly2: Record<'a', string> = { a: 'string' };

Compatibility

• TSLint: ban-types