When declaring properties or methods in a class or interface with an index signature, it is important to ensure that the type is compatible with what is specified in the index. This is why adding Function to the index signature can be beneficial.
The reasoning behind this is detailed in the documentation:
String index signatures are useful for describing the “dictionary” pattern, but they also require all properties to match their return types. For example, if the type of 'name' does not align with the string index’s type, it will result in a type-checker error:
interface NumberDictionary {
[index: string]: number;
length: number; // valid, as length is a number
name: string; // invalid, as 'name' is not a subtype of the indexer
}
Utilizing any in the indexer signature may not be advisable because it bypasses type-checking. Instead, using Function accurately reflects the data within the class. This is evident when accessing values through indexed access like so:
const key = 'greeting';
const value = this[key];
If the value of key happens to be 'greet', you might retrieve a function as the value. Similarly, assigning a string value to greet:
this['greet'] = 'hi!';
Will overwrite the method with a string value, making it inaccessible for calling.
Given these considerations, it is recommended to store dictionaries with index signatures in a separate property of the class rather than directly in the class itself. A structure like the following could be effective:
class Greeter {
data: { [key: string]: string | number[] } = {};
get greeting(): string { return this.data.greeting.toString() }
set greeting(value: string) { this.data.greeting = value };
constructor(message: string) {
this.greeting = message;
}
greet(): string {
return "Hello, " + this.greeting;
}
}