为什么我从NG-节目获得不同的结果= QUOT;！emptyArray＆QUOT;和NG-隐藏=＆QUOT; emptyArray＆QUOT;？ [英] Why am I getting different results from ng-show="!emptyArray" and ng-hide="emptyArray"?

问题描述

我一直以为 ngShow 和 ngHide 充当布尔对口对方。这种信念，然而，是由意外的行为动摇了 ngShow 当空数组参与。

I have always thought ngShow and ngHide act as boolean counterpart to each other. That belief, however, is shaken by the unexpected behaviour of ngShow when an empty array is involved.

下面是一个演示plunker 。为什么不是 NG-秀=！emptyArray行为就像 NG-隐藏=emptyArray？

Here is a demo plunker. Why isn't ng-show="!emptyArray" behaving like ng-hide="emptyArray"?

推荐答案

由于 []！==虚假。您可以强制长度值布尔与 !! 代替。

Because [] !== false. You can coerce the length value to boolean instead with !!.

<div ng-hide="!!emptyArray.length">emptyArray is falsy, so do not hide this.</div>
<div ng-show="!!!emptyArray.length">!emptyArray is truthy, so show this.</div>

编辑：

AngularJS的指令隐藏或显示取决于函数 toBoolean（）评估中传递的值。下面是的 toBoolean（）：

AngularJS's directive hide or show depends on the function toBoolean() for evaluating the value passed in. Here is the source code of toBoolean():

function toBoolean(value) {
  if (value && value.length !== 0) {
    var v = lowercase("" + value);
    value = !(v == 'f' || v == '0' || v == 'false' || v == 'no' || v == 'n' || v == '[]');
  } else {
    value = false;
  }
  return value;
}

和您可以验证在JS控制台下面code：

And you can verify the following code in JS console:

>var emptyArray = [];
>toBoolean(emptyArray)
false
>toBoolean(!emptyArray)
false

这解释了为什么。由于当 emptyArray 传递给 toBoolean（）直接，它的计算结果正确的结果假。然而，当！emptyArray 传递给 toBoolean（），它不评估为真，因为！emptyArray 是假本身。

That explains why. Since when emptyArray is passed to the toBoolean() directly, it evaluates the correct result false. However when !emptyArray is passed to toBoolean(), it doesn't evaluate to true since !emptyArray is false itself.

希望它帮助。

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