Warns if a type assertion does not change the type of an expression (`no-unnecessary-type-assertion`)

This rule prohibits using a type assertion that does not change the type of an expression.

Rule Details

This rule aims to prevent unnecessary type assertions.

Examples of code for this rule:

const foo = 3;
const bar = foo!;

const foo = <3>3;

type Foo = 3;
const foo = <Foo>3;

type Foo = 3;
const foo = 3 as Foo;

function foo(x: number): number {
  return x!; // unnecessary non-null
}

const foo = <number>3;

const foo = 3 as number;

const foo = 'foo' as const;

function foo(x: number | undefined): number {
  return x!;
}

This rule optionally takes an object with a single property typesToIgnore, which can be set to a list of type names to ignore.

For example, with @typescript-eslint/no-unnecessary-type-assertion: ["error", { typesToIgnore: ['Foo'] }], the following is correct code":

type Foo = 3;
const foo: Foo = 3;

If you don't care about having no-op type assertions in your code, then you can turn off this rule.