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

[sturloly]

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];

[sturloly]

自我理解尝试，求大神确认下， Chrome 调试：

var x =1 声明
1 打印

var b = [[1+x],[2+x],[3+x]]; 声明
b[0] 打印
[2]

x =2 声明
2 打印

b[0] 声明
[2] 打印

b 尝试调用一下
b[0]
[2] 没解析

var b = [[1+x],[2+x],[3+x]]; //重新声明下
b[0]
[3] 重新解析元素表达式了



求问书上的那句话是想表达这个点么？

[xxxyyy]

var obj = {
    _foo: 0,
    get foo() {
        return this._foo++;
    },
};
var ary = [obj.foo, obj.foo, obj.foo];
console.log(ary);

[YuJianrong]

@sturloly 是的就是你 demo 的意思， array 每次运行到的时候才会求值然后构造 array ，所以运行时的结果不同同样的 array 表达式的值也不同。就是说这个意思。

[fancy967]

呃，这个不是很常见的情况吗，数组的初始化值是动态的，所以每次初始化的结果不一定相等

[sturloly]

谢谢各位，本来不理解的是如果只是 var x =[1,2,3] 那再次重复声明下 var x= [1,2,3,4]当然不一样是句废话。 那再次声明 var x=[1,2,3]的话那是全等的啊怎么会可能不一样。所以其实是这句话可能缺少了“在数组内部嵌套的元素数组中，有对象类型的量”这个条件来使之可能被动态修改。所以本菜踩坑少就纠结了