How does declaring a Map collection with the var keyword differ from initialization through assignment after declaration?

In Go, declaring a map collection with the var keyword creates a variable with a zero value, which is nil for a map that is not initialized and not ready for use. For example:

var m map[string]int
fmt.Println(m == nil) // true

To use such a map, it must be initialized, for example, with make or assignment:

m = make(map[string]int)
m["key"] = 1

If you declare and initialize a map immediately with an assignment, for example:

m := map[string]int{"key": 1}

then the map is ready for use with the specified elements.

Thus, var declares a variable with a zero value, and initialization with assignment immediately creates and fills the map.