To access properties of an object where the property name contains problematic characters like e.g. ., (), #, -, +, * , / or (space), use the square bracket [] notation and pass in the name of the property as a string:

element['2021-12-03'] // returns your array

I'd recommend restructuring your element such that:

element: {
   date: '2021-12-03',
   items: [....] // Array(12)....
}

then if you had something like an array of element you could find a specific date via:

my_element = elements.find(x => x.date === '2021-12-03')

You can restructure as @iamaword said. like this:

let data = {'2021-12-03': [1,2,3,4], '2021-11-03': [5,6,7,8]}

let elements = Object.entries(data).map(item => {
  return {date: item[0], items: item[1]} // Depends on you.
});

elements.forEach(element => {
 console.log(`${element.date}:${JSON.stringify(element.items)}`);
});