I found this expression in a project:
export class MyComponent implements OnInit {
public field: { [key: number] : string } = something
...
}
and I don't know what it means. Can anyone explain this and give me a reference, so I can learn this type of declarations?
field is an object type (because of the {}) that has a numeric index signature that returns a string. This means that an object assigned to the field can only have numeric keys and the values in the object must be of type string
let field: { [key: number]: string };
field = {
0: "A",
//"A": "A", // error key is not numeric
//0: 0, // error value is not a string
}
let a = field[0] //a is string
field = ["A", "B"] // arrays are valid as well
You can read more about it here
key and name are common choices, but any name will do{ [key: number] : string }
is an anonymous/inline type declaration, it translates to:
interface Anon {
[key: number]: string;
}
The brackets are declare, that the type of any undeclared additional property in that object has to be of type number and the value of that type must be a string.
{ 1: "foo", 2: "bar" } // valid
{ "1": "foo", "2": "bar" } // invalid
