JSON Marshalling empty slices to empty arrays in Go

Today I came across a small gotcha in Golang.

I’m building a small JSON service that returns an array of IDs. Sometimes it is acceptable for this array to be empty, and in turn the service should return an empty array.

On the Go side of things, I am using a slice of integers ([]int64) to represent my ids. When my slice was empty, the endpoint was returning {"ids":null} rather than the desired {"ids":[]}.

The reason for this, had to do with how my []int64 was initialized.

I was initializing with var ids []int64. This is in fact, a nil pointer, so it makes sense that when it is automatically json.Marshal‘d, it goes to null instead of [].

The solution was to initialize with ids := make([]int64], 0).

The difference in these two ways of initializing a variable can be seen on the go playground.