Loop variable capture in goroutines (pre-Go 1.22)

In Go versions before 1.22, goroutines launched inside a range loop that close over the loop variable all see the same final value of that variable by the time they execute.

Capture the loop variable explicitly by passing it as a function argument or shadowing it:

// WRONG (pre-1.22) — all goroutines print the last value of v
for _, v := range items {
    go func() {
        fmt.Println(v) // captures v by reference
    }()
}

// RIGHT — shadow v inside the loop
for _, v := range items {
    v := v // new variable scoped to this iteration
    go func() {
        fmt.Println(v)
    }()
}

// ALSO RIGHT — pass as argument
for _, v := range items {
    go func(val string) {
        fmt.Println(val)
    }(v)
}

Go 1.22+ fixes this: each iteration creates a new loop variable automatically.

Before Go 1.22, a single loop variable is reused for every iteration. Closures capture the variable address, so all goroutines share the same memory location which holds the final value after the loop ends.