A type assertion provides access to an interface value's underlying concrete value.
```
t := i.(T)
```
This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.
If i does not hold a T, the statement will trigger a panic.
To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.
```
t, ok := i.(T)
```
If i holds a T, then t will be the underlying value and ok will be true.
If not, ok will be false and t will be the zero value of type T, and no panic occurs.
```
var i interface{} = "hello"
s := i.(string)
fmt.Println(s)
s, ok := i.(string)
fmt.Println(s, ok)
```