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

Nil slice — zero value of []T; pointer is nil, length and capacity are 0. Empty slice — has a backing array but no elements; length is 0 but capacity may be > 0.

Both support the same operations: len, cap, range, and append. The practical difference appears in JSON marshaling and equality checks.

1var nilS []int        // nil slice
2emptyS := []int{}     // non-nil, empty
3madeS := make([]int, 0, 5)  // non-nil, empty, cap=5
4
5// append works on all three
6nilS = append(nilS, 1)    // [1]
7emptyS = append(emptyS, 2) // [2]
8madeS = append(madeS, 3)   // [3]

JSON serialization:

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

When to use:

• Nil — default uninitialized state; no memory allocated.
• Empty — when API contracts require [] instead of null in JSON.

Common mistakes:

• Returning nil from REST APIs — clients expect [].
• Checking only len == 0 when nil vs empty matters.
• Assuming append(nilSlice, ...) panics — it doesn't.
• Comparing slices.Equal(nilS, emptyS) — they are not equal despite same length.