Have you come across this function declaration recently?
static required(control: AbstractControl): {
[key: string]: boolean;
};
What does the return value of this function mean? It seems to be returning an object with multiple properties, each containing a boolean value and a string name. This question is more related to TypeScript, but for those curious about where it's from - it can be found in Angular's Validators class.
This particular structure follows the concept of index signatures in typescript, where a key is associated with a value (previously referred to as indexable Types).
In this case, the keys are represented by strings and the corresponding values are booleans. Here's an example:
let map : { [key: string]: boolean} = {};
map["foo"] = true;
map["bar"] = false;
map.foo = true;
map["foobar"] = "foo"; // This line throws an exception
map[1] = true; // Interestingly, this does not throw an exception
Feel free to experiment with this code snippet on the Typescript Playground.
You can also express this format using the Record utility type which was first introduced in TypeScript version 2.1.
Record<Keys, Type>
Creates an object type with property keys defined by Keys and property values specified by Type. This handy tool allows for property mappings from one type to another.
The declaration {[key: string]: boolean;} is essentially the same as Record<string, boolean>.
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