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

Nil slice — zero value of []T, no backing array, pointer/length/capacity all zero. Empty slice — has a backing array but no elements; len == 0, cap may be > 0.

Both support len, cap, range, and append identically. The difference is in identity checks and serialization.

1var nilS []int        // nil
2emptyS := []int{}     // non-nil, empty
3madeS := make([]int, 0, 10)  // non-nil, empty, cap=10
4
5fmt.Println(nilS == nil)     // true
6fmt.Println(emptyS == nil)   // false
7fmt.Println(madeS == nil)    // false
8
9fmt.Println(len(nilS), cap(nilS))     // 0 0
10fmt.Println(len(emptyS), cap(emptyS))  // 0 0
11fmt.Println(len(madeS), cap(madeS))    // 0 10

JSON behavior:

1n, _ := json.Marshal(nilS)   // "null"
2e, _ := json.Marshal(emptyS) // "[]"

When to use:

• Nil — default uninitialized state; no memory used.
• Empty — API responses that must return [] not null.

Common mistakes:

• Returning nil from API handlers — JSON shows null.
• Checking only len == 0 when nil vs empty matters.
• Thinking append(nilSlice, ...) panics — it's safe.
• slices.Equal(nilS, emptyS) returns false even though both are empty.