Javascript数组非未定义元素计数 [英] Javascript array non undefined element count

问题描述

我用 let arr = new Array(99999) 创建了一个数组，但我没有将它填充到 arr.length，即 99999，我怎么知道这个数组中有多少实际的、非 undefined 元素?

I create an array with let arr = new Array(99999) but I don't fill it up to arr.length which is 99999, how can I know how much actual, non undefined elements do I have in this array?

有没有比查找第一个 undefined 更好的方法?

Is there a better way than to look for the first undefined?

推荐答案

你可以使用 Array#forEach，跳过稀疏元素.

You could use Array#forEach, which skips sparse elements.

let array = new Array(99999),
    count = 0;

array[30] = undefined;

array.forEach(_ => count++);

console.log(count);

与 Array 相同#reduce

let array = new Array(99999),
    count = 0;

array[30] = undefined;

count = array.reduce(c => c + 1, 0);

console.log(count);

为了过滤非稀疏/密集元素，你可以使用一个回调，它为每个元素返回 true.

For filtering non sparse/dense elements, you could use a callback which returns for every element true.

也许这个链接有助于理解稀疏数组的机制:JavaScript:稀疏数组 vs. 密集数组.

Maybe this link helps a bit to understand the mechanic of a sparse array: JavaScript: sparse arrays vs. dense arrays.

let array = new Array(99999),
    nonsparsed;

array[30] = undefined;

nonsparsed = array.filter(_ => true);

console.log(nonsparsed);
console.log(nonsparsed.length);

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