Skip to content
Biome
Docs Blog Playground

noBannedTypes

Diagnostic Category: lint/complexity/noBannedTypes

Since: v1.0.0

Sources:

Disallow primitive type aliases and misleading types.

  • Enforce consistent names for primitive types

Primitive types have aliases. For example, Number is an alias of number. The rule recommends the lowercase primitive type names.

  • Disallow the Function type

The Function type is loosely typed and is thus considered dangerous or harmful. Function is equivalent to the type (...rest: any[]) => any that uses the unsafe any type.

  • Disallow the misleading non-nullable type {}

In TypeScript, the type {} doesn’t represent an empty object. It represents any value except null and undefined. The following TypeScript example is perfectly valid:

const n: {} = 0
code-block.ts:1:10 lint/complexity/noBannedTypes ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Don’t use ’{}’ as a type.
  
  > 1 │ const n: {} = 0
      │          ^^
    2 │ 
  
  ℹ Prefer explicitly define the object shape. ’{}’ means “any non-nullable value”.

To represent an empty object, you should use { [k: string]: never } or Record<string, never>.

To avoid any confusion, the rule forbids the use of the type {}, except in two situations:

  1. In type constraints to restrict a generic type to non-nullable types:
function f<T extends {}>(x: T) {
    assert(x != null);
}
  1. In a type intersection to narrow a type to its non-nullable equivalent type:
type NonNullableMyType = MyType & {};

In this last case, you can also use the NonNullable utility type:

type NonNullableMyType = NonNullable<MyType>;

Examples

Section titled Examples

Invalid

Section titled Invalid
let foo: String = "bar";
code-block.ts:1:10 lint/complexity/noBannedTypes  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Don’t use ‘String’ as a type.
  
  > 1 │ let foo: String = “bar”;
      │          ^^^^^^
    2 │ 
  
  ℹ Use lowercase primitives for consistency.
  
  ℹ Safe fix: Use ‘string’ instead
  
    1   │ - let·foo:·String·=·“bar”;
      1 │ + let·foo:·string·=·“bar”;
    2 2 │   
  
let bool = true as Boolean;
code-block.ts:1:20 lint/complexity/noBannedTypes  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Don’t use ‘Boolean’ as a type.
  
  > 1 │ let bool = true as Boolean;
      │                    ^^^^^^^
    2 │ 
  
  ℹ Use lowercase primitives for consistency.
  
  ℹ Safe fix: Use ‘boolean’ instead
  
    1   │ - let·bool·=·true·as·Boolean;
      1 │ + let·bool·=·true·as·boolean;
    2 2 │   
  
let invalidTuple: [string, Boolean] = ["foo", false];
code-block.ts:1:28 lint/complexity/noBannedTypes  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ Don’t use ‘Boolean’ as a type.
  
  > 1 │ let invalidTuple: [string, Boolean] = [“foo”, false];
      │                            ^^^^^^^
    2 │ 
  
  ℹ Use lowercase primitives for consistency.
  
  ℹ Safe fix: Use ‘boolean’ instead
  
    1   │ - let·invalidTuple:·[string,·Boolean]·=·[“foo”,·false];
      1 │ + let·invalidTuple:·[string,·boolean]·=·[“foo”,·false];
    2 2 │

Valid

Section titled Valid
let foo: string = "bar";
let tuple: [boolean, string] = [false, "foo"];
Section titled Related links