Fix

The panic occurs because your code attempts to send data on a channel that has already been closed. You must ensure the channel remains open before sending or close it only after all sends are complete.

// Correct: Close channel only after all sends are done
func worker(ch chan int) {
    defer close(ch)
    for i := 0; i < 10; i++ {
        ch <- i
    }
}

// Incorrect: Closing while another goroutine might still send
// func badWorker(ch chan int) {
//     close(ch) // Panic if another goroutine sends here
//     ch <- 1   // PANIC: send on closed channel
// }

The "send on closed channel" error in Go happens when you try to put a message into a mailbox that has already been locked shut. In Go, a channel is a pipe for communication between threads; once you close it, no more data can be sent through it. You must finish sending all your data before you close the channel to avoid this crash.