how to assign a variable to a json field in javascript code example

Example: set variable with json javascript

var sammy = {
  "first_name"  :  "Sammy",
  "last_name"   :  "Shark",
  "online"      :  true
}
sammy.first_name --> "Sammy"
sammy.last_name  --> "Shark"
sammy.online     -->  true