The Loop Variable Capture Gotcha in Go (Pre-1.22)

In Go versions prior to 1.22, loop variables in for loops are reused across iterations, causing closures to capture the same variable instance rather than a copy of its value at that iteration. To fix this, explicitly assign the loop variable to a new local variable inside the loop body before using it in a closure.

for i := 0; i < len(items); i++ {
    index := i // Create a new variable for this iteration
    go func() {
        fmt.Println(items[index]) // Captures the correct value
    }()
}

Before Go 1.22, the loop variable is like a single sticky note that gets overwritten every time the loop runs. If you try to save that note for later use (like in a background task), all your saved notes end up showing the final value instead of the value from when they were created. You fix this by writing the value onto a fresh piece of paper inside the loop before you save it.