javascript 如果存在则使用值，否则使用 or 运算符分配默认值
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Use value if it exists, else assign default using the or operator
提问by Xiphias
I found this example in a book:
我在一本书中找到了这个例子：
// Create _callbacks object, unless it already exists
var calls = this._callbacks || (this._callbacks = {});
I simplified it so that I did not have to use a special object scope:
我简化了它，这样我就不必使用特殊的对象范围：
var a = b || (b = "Hello!");
When b is defined, it works. When b is not defined, it does not work and throws a ReferenceError.
当 b 被定义时，它就起作用了。当 b 未定义时，它不起作用并抛出一个 ReferenceError。
ReferenceError: b is not defined
Did I do anything wrong? Thank you!
我做错了什么吗？谢谢！
回答by Andrew Clark
When performing a property lookup like this._callback, if the _callbacksproperty does not exist for thisyou will get undefined. However if you just do a lookup on a bare name like b, you will get a reference error if bdoes not exist.
执行类似的属性查找时this._callback，如果该_callbacks属性不存在，this您将获得undefined. 但是，如果您只是在像 一样的裸名称上进行查找，b如果b不存在，您将收到引用错误。
One option here is to use a ternary with the typeofoperator, which will return "undefined"if the operand is a variable that has not been defined. For example:
这里的一种选择是将三元与typeof运算符一起使用，"undefined"如果操作数是尚未定义的变量，它将返回。例如：
var a = typeof b !== "undefined" ? b : (b = "Hello!");
回答by KooiInc
It should work in this form:
它应该以这种形式工作：
var b, a = b || (b = "Hello!", b);
// ^ assign b
// ^ () and , for continuation
// ^ return the new value of b
//=> result: a === b = "Hello!"
