为什么我从 ng-show="!emptyArray" 得到不同的结果?和 ng-hide="emptyArray"? [英] Why am I getting different results from ng-show="!emptyArray" and ng-hide="emptyArray"?

问题描述

我一直认为 ngShow 和 ngHide 是彼此对应的布尔值.然而，当涉及空数组时，ngShow 的意外行为动摇了这种信念.

这是一个演示工具.为什么 ng-show="!emptyArray" 的行为不像 ng-hide="emptyArray"?

解决方案

因为[] !== false.您可以使用 !! 将长度值强制为 boolean.

emptyArray 是假的，所以不要隐藏它.</div><div ng-show="!!!emptyArray.length">!emptyArray 是真的，所以显示这个.</div>

已

AngularJS 的指令 hide 或 show 依赖于函数 toBoolean() 来评估传入的值.这里是 toBoolean() 的源代码a href="https://github.com/angular/angular.js/blob/master/src/Angular.js#L835">toBoolean():

function toBoolean(value) {if (value && value.length !== 0) {var v =小写("+值)；value = !(v == 'f' || v == '0' || v == 'false' || v == 'no' || v == 'n' || v == '[]');} 别的 {值 = 假；}返回值；}

并且可以在JS控制台验证如下代码:

>var emptyArray = [];>toBoolean(emptyArray)错误的>toBoolean(!emptyArray)错误的

这就解释了原因.由于当 emptyArray 直接传递给 toBoolean() 时，它会评估正确的结果 false.但是，当 !emptyArray 传递给 toBoolean() 时，它不会计算为 true，因为 !emptyArray 是false 本身.

希望有帮助.

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.

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

解决方案

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>

Edited:

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;
}

And you can verify the following code in JS console:

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

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.

Hope it helps.

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