//// [implicitAnyGenericTypeInference.ts] interface Comparer { compareTo(x: T, y: U): U; } var c: Comparer; c = { compareTo: (x, y) => { return y; } }; var r = c.compareTo(1, ''); //// [implicitAnyGenericTypeInference.js] var c; c = { compareTo: function (x, y) { return y; } }; var r = c.compareTo(1, '');