javascript array - what is true expression?

Question

create 3 undefined, empty array.

var a1 = [,,,];
var a2 = new Array(3);

from JavaScript: The Definitive Guide,

0 in a1 //true
0 in a2 //false

but, in real world browser, getting different result. (IE8 and chrome 33...)

0 in a1 //false
0 in a2 //false

which is true, book or real world?

Answer 1

Looks like the book is wrong. As you can see from the specification, [,,,] does not add any values to the array:

The production ArrayLiteral : [ Elision_opt ] is evaluated as follows:

1. Let array be the result of creating a new object as if by the expression new Array() where Array is the standard built-in constructor with that name.
2. Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
3. Call the [[Put]] internal method of array with arguments "length", pad, and false.
4. Return array.

("Elisions" are the , which are not preceded or followed by an expression.)

In simpler terms:

• Create an empty array
• Evaluate the ,, which is basically just counting them, starting from 1. So ,,, results in 3.
• Then set the length of the array to the result (e.g. 3).

And that's exactly what new Array(3) is doing as well.

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There is also a less formal description of elided elements:

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.