无法分配给“布尔”，因为它是常量或只读属性

Question

I am creating a typescript object to represent read/write permissions in my app, but I came across this quirk in assigning the variables that makes no sense to me. Hopefully, someone can look at this and tell me what I am doing wrong.

export class Permission {
       watcher:Object =  { read:Boolean = true};
}

The previous code results in a error which says:

[ts] Cannot assign to 'Boolean' because it is a constant or a read-only property. const Boolean: BooleanConstructor

I'm probably just doing something stupid, so apologies in advance if this is super newbish.

Answer 1

If you intended to type your watcher property to be an object with property read , then it should be typed something like this:

export class Permission {
  // The part after the ":" is the type definition of the property. 
  // The part after "=" is its value
  watcher: { 
    read: boolean
  } = { read: true };
}

What you currently have is probably a syntactic error. You probably do not want to override the built-in constructor Boolean with Boolean = true

Answer 2

You are trying to assign a JSON object with { read: true } , declaring it as type Boolean is just redundant (because true and false are always Boolean) typescript infers the type, and it's an invalid JSON structure.

JSON has this format:

{
  key: value,
  key2: value2
}

Notice there are no equal signs ( = ) in a JSON, only : to separate the keys and values.

Now I see the class syntax is confusing you, because it does have an equal sign, but It's only for the assignment. After the assignment you don't need to have an equals sign.

Your final class definition should look something like this:

export class Permission {
    watcher = { read: true };
}

You would need interfaces for more complex objects, in case you want optional variables or dependencies on other objects. For a small object typescript will understand the structure by the types.