A custom property has a name and an optional (string) value.
一个自定义属性有一个名称和一个可选值(字符串)。
For example, besides house number and street name, the Address object has one more optional property as Cross street which is not shown in the above code segment.
例如，除了门牌号和街道名称，Address对象还有几个可选属性，如Cross Street，它在上面的代码片段中并没有用到。
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