Can I limit the length of an array in JavaScript?

You're not using splice correctly:

arr.splice(4, 1)

this will remove 1 item at index 4. see here

I think you want to use slice:

arr.slice(0,5)

this will return elements in position 0 through 4.

This assumes all the rest of your code (cookies etc) works correctly

arr.length = Math.min(arr.length, 5)

The fastest and simplest way is by setting the .length property to the desired length:

arr.length = 4;

This is also the desired way to reset/empty arrays:

arr.length = 0;

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Caveat: setting this property can also make the array longer than it is: If its length is 2, running arr.length = 4 will add two undefined items to it. Perhaps add a condition:

if (arr.length > 4) arr.length = 4;

Alternatively:

arr.length = Math.min(arr.length, 4);