How to create Map<String,List> in Javascript? Push method does not seem to work if we define aMap = {} and if(aMap [key]) {
aMap[key].push(value);//this is not working for some reason }

How to create Map<String,List<String>> in Javascript? Push method does not seem to work if we define

aMap = {} and 
if(aMap [key])
{                                      
    aMap[key].push(value);//this is not working for some reason
}