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What is “undefined x 1” in JavaScript?

In Chrome 21, feeding [,] to the console outputs

[undefined x 1]

and feeding [undefined] outputs

[undefined]

What is the difference between [undefined] and [undefined x 1]?

What is the notation [undefined x 1]?

Answer 1

[,] is a sparse array. It has a length of 1, but no values (0 in [,] === false). It can also be written as new Array(1).

[undefined] is an array of length 1 with the value undefined at index 0.

When accessing the property “0“, both will return undefined – the first because that property is not defined, the second because the value is “undefined”. However, the arrays are different, and so is their output in the console.