JavaScript Weirdness

Okay, in case you didn't know already, Javascript is a funky language.

I am currently working on a simple calendar web app using CSS Grid and a bit of react, so I wanted to generate an array of arrays storing the days in each month in each year. I was hoping to do this with some fancy forEach loops so I started with the following code:

1const year = (new Array(12)).fill([]);

1const year = (new Array(12)).fill([]);

I was hoping this would return something like this:

1year = [[], [], [], [], [], [], [], [], [], [], [], []];

1year = [[], [], [], [], [], [], [], [], [], [], [], []];

but it instead returned essentially this:

1year = [12 x []];

1year = [12 x []];

which basically boils down to an array of 12 pointers all pointing to the same virtual array. It took about 45 minutes debugging this with one of my friends because it kept returning an array of 12 arrays each holding 365 items.

The way that I got around this issue was by setting year to:

1const year = [[], [], [], [], [], [], [], [], [], [], []];

1const year = [[], [], [], [], [], [], [], [], [], [], []];

Then I could add in all my days within each array.