Type Assertion

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 Tvalue 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.

Note the similarity between this syntax and that of reading from a map.

For example:

package main

import "fmt"

func main() {
    var i interface{} = "hello"
   
    s := i.(string)
    fmt.Println(s)

    s, ok := i.(string)
    fmt.Println(s, ok)

    f, ok := i.(float64)
    fmt.Println(f, ok)

    f = i.(float64) // panic
    fmt.Println(f)
}

Output is:

hello
hello true
0 false
panic: interface conversion: interface {} is string, not float64

Reference: https://tour.golang.org/methods/15