What is "undefined x 1" in JavaScript?

Question

I'm doing some small experiments based on this blog entry.

I am doing this research in Google Chrome's debugger and here comes the hard part.

I get the fact that I can't delete local variables (since they are not object attributes). I get that I can 'read out' all of the parameters passed to a function from the array called 'arguments'. I even get it that I can't delete and array's element, only achieve to have array[0] have a value of undefined.

Can somebody explain to me what undefined x 1 means on the embedded image?

And when I overwrite the function foo to return the arguments[0], then I get the usual and 'normal' undefined.

This is only an experiment, but seems interresting, does anybody know what undefined x 1 refers to?

Solution 1

That seems to be Chrome's new way of displaying uninitialized indexes in arrays (and array-like objects):

> Array(100)
[undefined × 100]

Which is certainly better than printing [undefined, undefined, undefined,...] or however it was before.

Although, if there is only one undefined value, they could drop the x 1.

Solution 2

Newer versions of Chrome use empty instead of undefined to make the difference a bit clearer.

For example, entering [,,undefined,,] in the DevTools console results in:

▼(4) [empty × 2, undefined, empty]
   2: undefined
   length: 4
  ▶__proto__: Array(0)

Solution 3

Google Chrome seems to choose to display sparse array using this undefined x n notation. It will show [undefined, undefined] if it is not a sparse array:

var arr = new Array(2);
console.log(arr);

var arr2 = [];
arr2[3] = 123;
console.log(arr2);

var arr3 = [,,,];
console.log(arr3)

var arr4 = [,];
console.log(arr4)

var arr5 = [undefined, undefined]
console.log(arr5)

var arr6 = [undefined]
console.log(arr6)

arr1 to arr4 are all sparse arrays, while arr5 and arr6 are not. Chrome will show them as:

[undefined × 2] 
[undefined × 3, 123] 
[undefined × 3] 
[undefined × 1] 
[undefined, undefined] 
[undefined] 

note the [undefined x 1] for the sparse array.

Since you deleted an element, it follows that: If you delete an element from an array, the array becomes sparse.

Solution 4

Here is how it looks, Type this code into chrome devtools console:-

arr = new Array(4);
arr[2] = "adf";
console.log(arr);    // [undefined × 2, "adf", undefined × 1]

See the first two consecutive "undefined" that are represented by *2, to show that there are two consecutive undefined there

View more solutions