Problem 1: markedDates has been defined as an array ([]). To define it as an object (which is what you have as your expected output), use {}. (No such thing as an 'associative array' in JavaScript; arrays use 0, 1, etc. as indexes. Objects use 0, 1, key, another_key as indexes [or keys, more accurately].)

Problem 2: You haven't defined the dates array within markedDates which will cause a reference error.

Problem 3: You are pushing strings to the array instead of objects. This won't error, but won't produce the intended output.

Problem 4: You're trying to use styleClass as an array - it's just a string property within the markedDates.

To fix your code in the line-by-line manner (keeping similarity to your original code):

var markedDates = {};

markedDates.dates = [];
markedDates.dates.push({day: 1, month: 12, year: 2021});
markedDates.dates.push({day: 15, month: 12, year: 2021});
markedDates.styleClass = "test";

console.log(markedDates);

.as-console-wrapper { max-height: 100% !important; top: auto; }

But if you've got your expected output statically defined at the top, you can just assign that directly to the variable as such:

var markedDates = {
  dates: [{ day: 1, month: 12, year: 2021 }, { day: 15, month: 12, year: 2021 }],
  styleClass: 'test'
};

console.log(markedDates);

Things that you should consider -

1. Make an object that needs to be pushed into the bigger array. Eg: markedDate in the below code.
2. Assigning a string should be directly with an equal to operator and no need to push it, like in an array
3. Initialize the internal field dates as an array before pushing into it first

var markedDates = [];
var markedDate = {};
markedDate['dates'] = [];
markedDate['dates'].push({day: 1, month: 12, year: 2021});
markedDate['dates'].push({day: 15, month: 12, year: 2021});
markedDate['styleClass'] = 'test';
markedDates.push(markedDate);
console.log(markedDates);