Unexpectedly, the code below compiles without any errors (using tsc 3.9.5):
interface IDateHandler {
handleDate: (Date) => void;
}
let dateHandler: IDateHandler = {
handleDate: (d: Date) => {},
};
dateHandler.handleDate([1, 2, 3]);
Even more surprising is that if handleDate([1, 2, 3]) is replaced with handleDate([1, 2, 3], 4), tsc throws
error TS2554: Expected 1 arguments, but got 2.Clearly, it recognizes the function signature of handleDate, yet it seems to be disregarding the actual types.
Even more surprisingly, removing the interface declaration causes type checking to work correctly:
let dateHandler = {
handleDate: (d: Date) => {},
};
dateHandler.handleDate([1, 2, 3]);
// error TS2345: Argument of type 'number[]' is not assignable to parameter of type 'Date'.
// Type 'number[]' is missing the following properties from type 'Date': toDateString, toTimeString, toLocaleDateString, toLocaleTimeString, and 37 more.
What's happening here?