parse() 
For convert a JSON format to javascript objects

var obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}');
document.getElementById("demo").innerHTML = obj.name + ", " + obj.age;

//John, 30
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stringify() 
To convert a javascript Object to JSON format

var arr = [ "John", "Peter", "Sally", "Jane" ];
var myJSON = JSON.stringify(arr);
document.getElementById("demo").innerHTML = myJSON;

//John, 30