How to Check Whether a Map Contains a Key in Go

Go has a built-in idiom for checking whether a map contains a key. A map lookup can return both the stored value and a boolean indicating whether the key is present.

Check a Key

myMap := map[string]int{
	"apple":  1,
	"banana": 2,
	"cherry": 3,
}

key := "banana"
value, exists := myMap[key]

if exists {
	fmt.Printf("Key '%s' exists with value %d\n", key, value)
} else {
	fmt.Printf("Key '%s' does not exist\n", key)
}

The boolean is important because a missing key returns the value type’s zero value. Without the boolean, you cannot distinguish a missing key from a present key whose stored value happens to be zero.

Check Only for Presence

If you do not need the value, discard it with _:

key := "grape"
_, exists := myMap[key]

if !exists {
	fmt.Printf("Key '%s' does not exist\n", key)
}

A compact form is also common:

if value, ok := myMap[key]; ok {
	fmt.Println(value)
}

Custom Key Types

Map keys may use any comparable type, including structs whose fields are all comparable:

type Person struct {
	Name string
	Age  int
}

myMap := map[Person]string{
	{Name: "Alice", Age: 30}: "Engineer",
	{Name: "Bob", Age: 25}:   "Artist",
}

person := Person{Name: "Alice", Age: 30}
value, exists := myMap[person]

if exists {
	fmt.Printf("Person '%v' exists with value '%s'\n", person, value)
}

The lookup pattern is exactly the same regardless of key type.

Conclusion

Use value, ok := m[key] whenever you need to know whether a key is actually present in a Go map. It is concise, avoids ambiguity around zero values, and is the standard Go idiom for map membership checks.