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?Used When checking whether the variable before the question mark is null or undefined, the program will not make an error.

stu2: Student = { id: 1, name: undefined, age: 3 }; 
stu2 name {{stu2!.name}}
 // stu2 name 
stu2 id {{stu2!.id}}
 // stu2 id 1

!Used to check that the program will not make an error when the variable after the exclamation mark is null or undefined.

stu3: Student; 
stu3 id{{stu3?.id}}
 // stu3 id 
stu3 name{{stu3?.name}}
 // stu3 name

For more programming-related knowledge, please visit:Introduction to Programming! !

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