As well as the type that is traveling through the channel, there is another property that the channel has: its capacity. This represents the number of items a channel can hold before any new attempt to send an item is made, resulting in a blocking operation. The capacity of the channel is decided at its creation and its default value is 0:

// channel with implicit zero capacity
var a = make(chan int)

// channel with explicit zero capacity
var a = make(chan int, 0)

// channel with explicit capacity
var a = make(chan int, 10)

The capacity of the channel cannot be changed after its creation and can be read at any time using the built-in cap function:

func main() {
    var (
        a = make(chan int, 0)
        b = make(chan int, 5)
    )

    fmt.Println("a is", cap(a))
    fmt.Println("b is", cap(b))
}

The full example is available at https://play.golang.org/p/Yhz4bTxm5L8.

The len function, when used on a channel, tells us the number of elements that are held by the channel:

func main() {
    var (
        a = make(chan int, 5)
    )
    for i := 0; i < 5; i++ {
        a <- i
        fmt.Println("a is", len(a), "/", cap(a))
    }
}

The full example is available at https://play.golang.org/p/zJCL5VGmMsC.

From the previous example, we can see how the channel capacity remains as 5 and the length grows with each element.