JSON.stringify returns []

json-stringfy.js

function json_encode(){
    var content = [];
    content.field1 = { width: "100px", height: "400px", label : "Width"};
    content.field2 = { width: "200px", height: "500px", label : "Height"};
    console.log(JSON.stringify(content));
  }

// The above function return [] because the key used here is a string. 
//The array key should be integer for JSON.stringify to return proper value