Question

我正在尝试访问像这样的对象属性：

var scope1 = {a: {value: 25}};
var scope2 = undefined;

var v1 = scope1.b.value || 0;  // TypeError: Cannot read property 'value' of undefined
var v2 = scope2.b.value || 0;  // TypeError: Cannot read property 'value' of undefined

明显的||经营者没有给我我想要的东西。我知道我可以做以下事情

var vv2 = (scope1 && scope1.b ? scope1.b.value : 0);

但这会使代码变得非常冗长......所以任何人都知道有没有更简单的方法来做我想做的事情？感谢。

Answer 1

通常的答案是&&运算符：

var v1 = scope1.b && scope1.b.value || 0;

...因为与||一样，&&是curiously powerful：&&评估左侧操作数，如果它是假的，则采用该假名价值作为结果;如果左侧操作数值为真，则&&计算右侧操作数并将该值作为结果。

如果scope1.b是undefined：

// We start with
scope1.b && scope1.b.value || 0
// which is
undefined && scope1.b.value || 0
// which is
undefined || 0
// which is
0

...但如果scope.b是像对象引用那样的真值：

// We start with
scope1.b && scope1.b.value || 0
// which is
scope1.b.value || 0
// which is
scope1.b.value // *IF* scope1.b.value is truthy, or
0              // If it isn't

（回想一下，＆＃34; falsey＆＃34;值为undefined，null，0，""，NaN，当然，false;以及＆＃34; truthy＆＃34;值都是其他值。）

示例：

＆＃13;

var scope1;

scope1 = {a: "foo"};
snippet.log(scope1.b && scope1.b.value || 0); // 0

scope1 = {a: "foo", b: {}};
snippet.log(scope1.b && scope1.b.value || 0); // 0

scope1 = {a: "foo", b: {value: 42}};
snippet.log(scope1.b && scope1.b.value || 0); // 42

＆＃13;

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

＆＃13;

我不推荐它，但你可以编写一个处理任意深度的函数：

＆＃13;

function resolve(obj, path, defValue) {
  var rv = path.split(".").reduce(function(o, p) {
    return o && o[p];
  }, obj);
  return rv || defValue;
}

var scope1;

scope1 = {a: "foo"};
snippet.log(resolve(scope1, "b.value", 0)); // 0

scope1 = {a: "foo", b: {}};
snippet.log(resolve(scope1, "b.value", 0)); // 0

scope1 = {a: "foo", b: {value: 42}};
snippet.log(resolve(scope1, "b.value", 0)); // 42

scope1 = {a: "foo", b: {}};
snippet.log(resolve(scope1, "b.baz.boz.value", 0)); // 0

scope1 = {a: "foo", b: {baz: {boz: {value: 67}}}};
snippet.log(resolve(scope1, "b.baz.boz.value", 0)); // 67

＆＃13;

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

＆＃13;

该实现不支持括号表示法，只是点符号，但如果您有所需的数字索引，则可以自由编写resolve(obj, "foo.0.bar.42", 0)，因为它不受标准JavaScript解析规则的约束。（或者将其扩展到支持[]并不会那么难。）

Javascript获取对象属性和子属性

1 个答案: