Go: What is the difference between nil and empty slice in Go?

nil slice — no underlying array. Empty slice — has underlying array.

Nil slice:

• No underlying array.
• Length = 0, capacity = 0.
• Zero value for slice.

1var s []int  // nil slice
2fmt.Println(s == nil)  // true
3fmt.Println(len(s))    // 0
4fmt.Println(cap(s))    // 0

Empty slice:

• Has underlying array.
• Length = 0, capacity > 0.
• Created with make or append.

1s := make([]int, 0)  // Empty slice
2s := []int{}        // Empty slice
3s := []int{1, 2, 3}[0:0]  // Empty slice
4
5fmt.Println(s == nil)  // false
6fmt.Println(len(s))    // 0
7fmt.Println(cap(s))    // > 0

JSON marshaling:

• nil slice → null.
• empty slice → [].

Best practice:

• Use empty slice for JSON.
• Use nil to distinguish uninitialized.