Array of undefined values

Question

I am not able to understand if there is an array of empty values as:

let arr=[,,,];

and I try to log the length then I get 3 instead of 4

let arr=[,,,];
console.log('length',arr.length);
console.log('arr[1]',arr[1]);

• j08691: Is your question why length reports three instead of four, or something else?
• Varun Sukheja: yeah it's about length

Answer 1

Trailing commas are optional in array literals (in modern JS, at least). When you have

[<valid array list>,]

this is equivalent to

[<valid array list>]

So

[,,,]

is like

[undefined,undefined,undefined,]

The specification calls this construction [ ElementList , Elision(opt) ]

where Elision is the optional trailing comma.

If an element is elided at the end of an array, that element does not contribute to the length of the Array.