角度表达
实际上，这不是特别的，点击ng，这是角度表达式的默认行为。
buggy()不使用常规javascript进行评估。它使用$ parse进行评估。
$ parse计算表达式并返回一个可以对范围运行的函数。
$ parse只有当表达式无效时才会记录错误。
角度表达评估是宽容的，我想不出有什么办法通过。
为什么表达是宽恕的？
Angular表达式被原谅的未定义和null的理由与数据绑定有关。当绑定变量被编译到DOM中时，绑定变量可能最初未定义或为空，当绑定变量取决于承诺时，真正明确的示例就是绑定变量。角色团队决定，除非出现错误信息，直到该承诺得到解决，否则最好继续默默地继续。
从Angular guide to expressions：
It makes more sense to show nothing than to throw an exception if a is
undefined (perhaps we are waiting for the server response,and it will
become defined soon). If expression evaluation wasn’t forgiving we’d
have to write bindings that clutter the code,for example:
{{((a||{}).b||{}).c}}
另请参见：https://groups.google.com/forum/m/#!topic/angular/HRVOUKEHLFw
Angular expressions：
Expressions are JavaScript-like code snippets that are usually placed in bindings such as {{ expression }}. Expressions are processed by the $parse service. Expressions are often post processed using filters to create a more user-friendly format.
Angular Expressions vs. JS Expressions
It might be tempting to think of Angular view expressions as JavaScript expressions,but that is not entirely correct,since Angular does not use a JavaScript eval() to evaluate expressions. You can think of Angular expressions as JavaScript expressions with following differences:
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Attribute Evaluation: evaluation of all properties are against the scope doing the evaluation,unlike in JavaScript where the expressions are evaluated against the global window.
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Forgiving: expression evaluation is forgiving to undefined and null,unlike in JavaScript,where trying to evaluate undefined properties can generate ReferenceError or TypeError.
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No Control Flow Statements: you cannot do any of the following in angular expression: conditionals,loops,or throw.
