Set up a map in js using json

How to set up a map (actually using a json implementation)

 
var a = {}; 
a["key1"] = "value1"; 
a["key2"] = "value2"; 

Since it's a map, there's a way to retrieve whether a key exists or not

 
if ("key1" in a) { 
// something 
} else { 
// something else 
} 

A simple one-sentence way to declare the key and value in the map:

 
var a = {'key1': 'value1', 'key2': 'value2'}