TypeScript使用切换大小写标识时出错(TS2367)

Question

我的逻辑工作方式不同，我打算或编译器不理解我想做什么?

Line i4:This condition will always return true since the types SubjectType.Both | SubjectType.A and SubjectType.B have no overlap.

j1  enum SubjectType {
j2      BOTH,
j3      A,
j4      B,
j5  }
i1  switch ( subjectType: SubjectType ) {
i2      case SubjectType.BOTH
i3      case SubjectType.A:
i4          if( subjectType !== SubjectType.B ) {
i5              //Do thing A
i6          }
i7      case SubjectType.B:
i8          if( subjectType !== SubjectType.A ) {
i9              //Do thing B
i10         }
i11 
i12         break;
i13      default:
i14          throw new TypeError( `Unknown SubjectType ${subjectType} provided` );
i15  }

编辑:事实上，我的逻辑是错误的。

i1  switch ( subjectType: SubjectType ) {
i2      case SubjectType.BOTH
i3      case SubjectType.A:
i4          //Do thing A, if type A | BOTH
i5
i6      case SubjectType.B:
i7          if( subjectType !== SubjectType.A ) {
i8              //Do thing B
i9          }
i10 
i11         break;
i12      default:
i13          throw new TypeError( `Unknown SubjectType ${subjectType} provided` );
i14  }

Answer 1

subjectType变量/const只能有三种状态:A,B和BOTH。考虑这个例子:

enum SubjectType {
BOTH,
A,
B,
}
const subjectType: SubjectType = null as any
function foo() {
switch (subjectType) {
case SubjectType.BOTH:
case SubjectType.A:
if (subjectType !== SubjectType.B) {
//Do thing A
}
case SubjectType.B:
if (subjectType !== SubjectType.A) {
//Do thing B
}
break;
default:
throw new TypeError(`Unknown SubjectType ${subjectType} provided`);
}
}

switch中的这两行:

case SubjectType.BOTH:
case SubjectType.A:

表示subjectType是BOTH或A中的一个，而不是B。

然后，你试图比较subjectType和Bif (subjectType !== SubjectType.B) {，但TS和我都清楚，在这种情况下，subjectType不能是B。