🔍 requires type information 🔧 fixable
Disallows type assertions that don't change the type or are not necessary in the first place.
Note that this rule marks all non-null assertions and definite assignment assertions as useless unless strictNullChecks (and strictPropertyInitialization for definite assignment assertion class properties) is enabled.
If a type assertion doesn't change the type of an expression, it just adds visual clutter. Even worse: type assertions hinder type inference.
In general you should try to infer types instead of asserting them.
👎 Examples of incorrect code
declare function takeString(param: string): void;
declare function takeStringOrUndefined(param?: string): void;
declare let str: string;
declare let optionalStr: string | undefined;
takeString(<string>str); // 'str' is already of type 'string'
takeString(str!); // non-null assertion is useless because 'str' is not nullable
takeStringOrUndefined(optionalStr!); // non-null assertion is unnecessary because the function accepts 'string | undefined'
takeStringOrUndefined(optionalStr as string); // same as above with type assertion syntax
let foo!: number | undefined; // definite assignment assertion is unnecessary as the type includes 'undefined'
let bar!: number;
bar!; // non-null assertion is unnecessary as 'bar' is definitely assigned
class C {
'prop'!: number; // definite assignment assertion is unnecessary as TypeScript doesn't check quoted property names
}
let tuple = [1 as const] as const; // inner 'const' assertion is redundant👍 Examples of correct code
declare function takeString(param: string): void;
declare let optionalStr: string | undefined;
takeString(<string>optionalStr);
takeString(optionalStr!);
let foo!: number;
let bar: number;
console.log(bar!); // non-null assertion suppresses 'Variable "bar" is used before being assigned.' error
class C {
prop!: number;
}