List of null variables

I'm having trouble grasping the concept of an array filled with empty values like this:

let arr=[,,,];

When I attempt to log the length, it shows 3 instead of 4.

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

Answer №1

Did you know that trailing commas are actually optional in array literals, at least in modern JS? For example, when you have:

[<a valid list of items>,]

this is the same as:

[<a valid list of items>]

Therefore, an array like this:

[,,,]

is essentially equivalent to:

[undefined, undefined, undefined,]

The official specification refers to this construction as [ ElementList , Elision(opt) ].

Just remember, if you leave out an element at the end of an array with a trailing comma, it won't count towards the length of the Array.

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