How to decode a JWT token in Go?

Function jwt.ParseWithClaims accept an interface of jwt.Claims as the second argument. Besides struct-based custom claims, the package also provides map-based claims, i.e. jwt.MapClaims.
So, you can simply decode the token into a MapClaims, e.g.

tokenString := "<YOUR TOKEN STRING>"    
claims := jwt.MapClaims{}
token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
    return []byte("<YOUR VERIFICATION KEY>"), nil
})
// ... error handling

// do something with decoded claims
for key, val := range claims {
    fmt.Printf("Key: %v, value: %v\n", key, val)
}