Mastering Go - Second Edition

About type assertions

A type assertion is the x.(T) notation, where x is an interface type and T is a type. Additionally, the actual value stored in x is of type T and T must satisfy the interface type of x. The following paragraphs, as well as the code example, will help you to clarify this relatively eccentric definition of a type assertion.

Type assertions help you to do two things. The first thing is checking whether an interface value keeps a particular type. When used this way, a type assertion returns two values: the underlying value and a bool value. Although the underlying value is what you might want to use, the Boolean value tells you whether the type assertion was successful or not.

The second thing that type assertions help with ...