Javascript 权威指南中的 Array Initializer 的问题求教

4.2 Object and Array Initializers Object and array initializers are expressions whose value is a newly created object or array. These initializer expressions are sometimes called “ object literals ” and “ array literals.” Unlike true literals, however, they are not primary expressions, because they include a number of subexpressions that specify property and element values. Array initializers have a slightly simpler syntax, and we ’ ll begin with those. An array initializer is a comma-separated list of expressions contained within square brackets. The value of an array initializer is a newly created array. The elements of this new array are initialized to the values of the comma-separated expressions: [] // An empty array: no expressions inside brackets means no elements [1+2,3+4] // A 2-element array. First element is 3, second is 7 The element expressions in an array initializer can themselves be array initializers, which means that these expressions can create nested arrays: var matrix = [[1,2,3], [4,5,6], [7,8,9]];

The element expressions in an array initializer are evaluated each time the array ini- tializer is evaluated. This means that the value of an array initializer expression may be different each time it is evaluated.

淘宝翻译版： JavaScript 对数组初始化表达式进行求值得时候，数组初始化表达式中的元素表达式也都会各自计算一次。也就是说，数组初始化表达式每次计算的值都有可能是不同的.

问题： 1 ，为什么不同？ 2 ，意味着什么？怎么样表现？有什么坑能举个栗子么?

Undefined elements can be included in an array literal by simply omitting a value be- tween commas. For example, the following array contains five elements, including three undefined elements: var sparseArray = [1,,,,5];