no-unnecessary-type-assertion
Disallow type assertions that do not change the type of an expression.
Extending "plugin:@typescript-eslint/recommended-type-checked"
in an ESLint configuration enables this rule.
Some problems reported by this rule are automatically fixable by the --fix
ESLint command line option.
This rule requires type information to run.
TypeScript can be told an expression is a different type than expected using as
type assertions.
Leaving as
assertions in the codebase increases visual clutter and harms code readability, so it's generally best practice to remove them if they don't change the type of an expression.
This rule reports when a type assertion does not change the type of an expression.
module.exports = {
"rules": {
"@typescript-eslint/no-unnecessary-type-assertion": "error"
}
};
Examples
- ❌ Incorrect
- ✅ Correct
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!;
}
Options
This rule accepts the following options
type Options = [
{
/** A list of type names to ignore. */
typesToIgnore?: string[];
},
];
const defaultOptions: Options = [{}];
typesToIgnore
With @typescript-eslint/no-unnecessary-type-assertion: ["error", { typesToIgnore: ['Foo'] }]
, the following is correct code":
type Foo = 3;
const foo: Foo = 3;
When Not To Use It
If you don't care about having no-op type assertions in your code, then you can turn off this rule.