This rule aims to standardize the use of type assertion style across the codebase.
Type assertions are also commonly referred as "type casting" in TypeScript (even though it is technically slightly different to what is understood by type casting in other languages), so you can think of type assertions and type casting referring to the same thing. It is essentially you saying to the TypeScript compiler, "in this case, I know better than you!".
type Options =
| {
assertionStyle: 'as' | 'angle-bracket';
objectLiteralTypeAssertions: 'allow' | 'allow-as-parameter' | 'never';
}
| {
assertionStyle: 'never';
};
const defaultOptions: Options = {
assertionStyle: 'as',
objectLiteralTypeAssertions: 'allow',
};
This option defines the expected assertion style. Valid values for assertionStyle
are:
as
will enforce that you always use... as foo
.angle-bracket
will enforce that you always use<foo>...
never
will enforce that you do not do any type assertions.
Most codebases will want to enforce not using angle-bracket
style because it conflicts with JSX syntax, and is confusing when paired with with generic syntax.
Some codebases like to go for an extra level of type safety, and ban assertions altogether via the never
option.
Always prefer const x: T = { ... };
to const x = { ... } as T;
(or similar with angle brackets). The type assertion in the latter case is either unnecessary or will probably hide an error.
The compiler will warn for excess properties with this syntax, but not missing required fields. For example: const x: { foo: number } = {};
will fail to compile, but const x = {} as { foo: number }
will succeed.
The const assertion const x = { foo: 1 } as const
, introduced in TypeScript 3.4, is considered beneficial and is ignored by this option.
Examples of incorrect code for { assertionStyle: 'as', objectLiteralTypeAssertions: 'never' }
(and for { assertionStyle: 'as', objectLiteralTypeAssertions: 'allow-as-parameter' }
)
const x = { ... } as T;
Examples of correct code for { assertionStyle: 'as', objectLiteralTypeAssertions: 'never' }
.
const x: T = { ... };
const y = { ... } as any;
const z = { ... } as unknown;
Examples of correct code for { assertionStyle: 'as', objectLiteralTypeAssertions: 'allow-as-parameter' }
.
const x: T = { ... };
const y = { ... } as any;
const z = { ... } as unknown;
foo({ ... } as T);
new Clazz({ ... } as T);
function foo() { throw { bar: 5 } as Foo }
If you do not want to enforce consistent type assertions.
- TSLint: no-angle-bracket-type-assertion
- TSLint: no-object-literal-type-assertion