no-unnecessary-type-annotation.md

no-unnecessary-type-annotation

Finds type annotations of variables and parameters that can be removed because the compiler can infer the type.

Known limitation: sometimes removing a type annotation can cause circular inference and therefore a compiler error.

Failing:

const foo: 1 = 1;
const foo: number = 1;
let foo: number = 1;
const arr: string[] = ["foo", "bar"];

const myVerboseSignature: (a: number) => number = function(a: number): number {
    return a;
}

declare function takeCallback(cb: (a: string) => void): void;
takeCallback((a: string) => a);

Options

"check-return-type"

Enables checking function return types. This option can cause false positives for (mutually) recursive functions.