Unknown: A Better any

The unknown type is half a specific explicit type and half the type any which allows everything. Declaring a variable as unknown allows us to set a wide variety of types without allowing unwanted access to properties or the value of a type. The following code demonstrates that a variable with type any can be assigned a string and then used with a function of the string type.

Later, the variable is assigned to a number that does not have substr function. However, TypeScript does not catch an attempt to invoke a function that does not exist.

Changing ...