Cannot create property on string

Empty Objects need to be individually created before their values are assigned. And use of const is not a good idea here, anyway, it's just my suggestion.

const birthdays = {};
var day = 123;
var id = 21;
var birthday = 2016;
// Loop -> Passing day, id and birthday
birthdays[day] = {};
birthdays[day][id] = birthday;

I had a similar error message TypeError: Cannot create property 'false' on boolean 'false' in Node 14.3.0 (ES 2019), using the new Javascript semicolon-less syntax, with a variable declaration and assignment followed by a destructuring assignment on the next line.

The code that caused the error:

var a = b ()
[c, d] = e ()

Solved by:

var a = b ()
;
[c, d] = e ()