I’ve been a bit confused for a while over when a map in Golang gets created with a value of nil (its zero value) and when it does not, so I’m writing this to help me remember.

Let’s look at various ways to initialize a map:

package main

import "fmt"

func main() {
    m1 := map[string]string{}     // initializes map
    m2 := make(map[string]string) // also initializes map
    var m3 map[string]string      // does NOT initialize the map! HERE BE DRAGONS!

    fmt.Println(m1 == nil) // false
    fmt.Println(m2 == nil) // false
    fmt.Println(m3 == nil) // true

    m1["foo"] = "bar"
    m2["foo"] = "bar"
    m3["foo"] = "bar" // panics because the map is NOT initialized, i.e., map is still nil

My takeaways from this are:

  • I should (always?) initialize map variables.
  • map[string]string{} and make(map[string]string) are equivalent unless you need to either (a) give your map some values when creating it or (b) set the size of the map.

Further reading: Which is the nicer way to initialize a map?