Lo-Dash sortBy以字符串格式的日期数组 [英] Lo-Dash sortBy array of dates in string format

问题描述

我想知道为什么lodash与纯JavaScript sort()相比，不对字符串数组中的日期进行排序.是预期的行为还是错误?

I'd like to know why lodash doesn't sort array of dates in string format as compared with plain javascript sort(). Is it expected behavior or a bug?

var array = ["2014-11-11", "2014-11-12", null, "2014-11-01", null, null, "2014-11-05"];

_.sortBy(array);
// ["2014-11-11", "2014-11-12", null, "2014-11-01", null, null, "2014-11-05"]

_.sortBy(array, function(value) {return new Date(value);});
// [null, null, null, "2014-11-01", "2014-11-05", "2014-11-11", "2014-11-12"]

array.sort();
// ["2014-11-01", "2014-11-05", "2014-11-11", "2014-11-12", null, null, null]

使用的版本:Lo-Dash v2.4.1 –现代版本.

Version used: Lo-Dash v2.4.1 – Modern build.

推荐答案

如果看看lodash代码，您可能会看到它是如何实现的.内部的函数_.sortBy使用本机Array.prototype.sort(请参见源).但是根不存在.更有趣的是将函数compareAscending作为回调传递给本机sort(

If you take a look to lodash code you may see how it's implemented. Function _.sortBy inside uses native Array.prototype.sort (see source). But the root is not there. More interesting is function compareAscending that is passed as a callback to native sort (source). So in a few words your

_.sortBy(array, function(value) {return new Date(value);});

转换为:

array.sort(function(a, b) {
    var aa = new Date(a),
        bb = new Date(b);

    if (aa !== bb) {
        if (aa > bb) { return 1; }
        if (aa < bb) { return -1; }
    }
    return aa - bb;
})

那么为什么null是开头呢?因为new Date(null)返回的Thu Jan 01 1970 01:00:00小于数组中的其他任何日期.

So why nulls are in the beginning? Because new Date(null) returns Thu Jan 01 1970 01:00:00 which is less than any other date in your array.

原生sort呢?根据规范(请参见此处) 默认的排序顺序是根据字符串Unicode代码点确定的.如果简单-本机sort会将项目转换为字符串并比较字符串.因此，本机排序就像:

What about native sort? According to spec (see here) The default sort order is according to string Unicode code points. If simply - native sort converts items to strings and compares strings. So native sort is smth like:

_.sortBy(array, function(value) {return value + ''; });

一旦'null'字符串总是比日期字符串更大"(例如'2014-11-11')-null将位于结果数组的末尾.

As soon as 'null' string is always "bigger" than date string (like '2014-11-11') - nulls will be in the tail of the result array.

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