type User = {
name: string;
};
const flag: any = false;
let list1: User;
list1 = flag ? flag : 1; // hope error
Why list1 can be 1, why not an error: Type '1' is not assignable to type 'User'
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1 Answer
You are using any in the annotation to flag. any is by definition assignable to any type and assignable from any type. This means that the result of the expression flag ? flag : 1 will be any (since one of the results of the ternary expression is any the type of the expression is any | 1 which will get reduced to any).
Generally avoid any. If you really don't know a type at compile time use unknown. In this case removing the annotation yields an error as expected:
type User = {
name: string;
};
const flag = false; // no annotation
let list1: User;
list1 = flag ? flag : 1; // err
answered May 21, 2019 at 8:35
Titian Cernicova-Dragomir
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4 Comments
Rob Rombouts
also a general remark, it is better to use
interface instead of type/class if you don't implement any methods on the interface u create (like domain objects with only the property definitions, or POJO in Java). It makes your class/type more lightweightTitian Cernicova-Dragomir
@RobRombouts
type and interface basically the same thing in this case. class is a different beast. But type and interface are both types that are erased at compile time (they differ in what can be done with them in the type system but for this simple example there is no practical difference)Rob Rombouts
thanks for clearing that up for me, the advantage of an interface over a type is that an interface can be extended, type cant, got confused with that. I always use interface for that reason.
Titian Cernicova-Dragomir
@RobRombouts types can be used as a base type for an interface. They can't extend other interfaces themselves (although intersections model this well enough). Types can't participate in merging so that extension avenue is closed. Either one will work fro simple cases, although I usually prefer interfaces too :)