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?. 对象深处的属性或与函数调用，若引用为空（null or undefined）, 返回 undefined
a || b => a ? a : b
a && b => !a ? a : b
a ?? b => a !== undefined && a!== null ?a:b
??= => a ??= b (a不为null和undefined时被赋值)
||= => a ||= b (a存在时返回a, 否则赋值为b)
!!
运算符
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