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

Nil slice — declared but never initialized; pointer, length, and capacity are all zero. Empty slice — has a backing array allocated but contains no elements; len == 0 but cap may be > 0.

Both behave identically for len, cap, range, and append. The distinction matters for JSON serialization and equality comparisons. A nil slice marshals to JSON null, while an empty slice marshals to [].

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 marshaling:

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

Safely appending to a nil slice works — Go allocates the backing array on the first append.

When to use:

• Nil slice — default uninitialized state; no memory allocated yet.
• Empty slice — API responses that must return [] not null; initializing from a known source.

Common mistakes:

• Returning nil from an HTTP handler — JSON response shows null instead of [], confusing clients.
• Checking only len(s) == 0 when nil vs empty matters.
• Thinking append(nilSlice, ...) panics — it doesn't, Go handles it safely.
• Using slices.Equal(nilS, emptyS) — returns false even though both are empty.