数组出界：与不确定，或长度检查比较？ [英] Array Out of Bounds: Comparison with undefined, or length check?

问题描述

这似乎是一个常见的​​JavaScript成语：

this seems to be a common javascript idiom:

function foo (array, index) {
    if (typeof array[index] == 'undefined')
        alert ('out of bounds baby');
}

，而不是更prevalent（在其他语言）并且从概念上简单的：

as opposed to the more prevalent (in other languages) and conceptually simpler:

function foo (array, index) {
    if (index >= array.length)
        alert ('boo');
}

据我所知，第一种情况还将为这对他们有'空白'阵列工作，但是是一个共同的足够的情况下，以保证成语？

I understand that the first case will also work for arrays which have 'gaps' in them, but is that a common enough case to warrant the idiom?

这促使这个问题的code样品可以看出<一个href=\"http://stackoverflow.com/questions/610406/javascript-equivalent-to-printf-string-format/4673436#4673436\">here.在这种情况下，使用函数内的'参数'变量的时候，是不是理智的假设，这将是一个连续的阵列？

The code sample that prompted this question can be seen here. In this case, when using the 'argument' variable inside a function, isn't it sane to assume that it will be a contiguous array?

推荐答案

唯一的正确的方法是检查该指数与长度。

The only correct way is to check the index vs. the length.

一个元素可以被分配值未定义。它仅仅是傻的在这里使用它的定点。 （可能还有其他的，有效的和可能重叠，原因检查不确定的，而不是一个出界检查 - 在其他问题code将present可以说是错误的结果，当值给定的arg是的真正的未定义）

An element may be assigned the value undefined. It is just silly to use it for a sentinel here. (There may be other, valid and possibly overlapping, reasons for checking for undefined, but not "for an out of bound check" -- the code in the other question will present arguably wrong results when the value of the given arg is really undefined.)

快乐编码。

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