Readonly Variable:
maxLength: Readonly<Number | number | String | string> = 1;
vs
Readonly-Typed Method:
maxLength(length: Number | number | String | string): Readonly<Number | number | String | string> {
var width: Readonly<Number | number | String | string> = length;
return width;
}
- What separates these two concepts?
- Can values be assigned to a Readonly function at runtime?
Readonly<T> defines an object-type mapping:
type Readonly<T> = {
readonly [P in keyof T]: T[P];
};
This feature ensures that all properties of object T are made read-only to the compiler - which means it is not suitable for use with primitive types like number or string since they do not have properties.
If you require a property that is read-only externally but can be changed internally at runtime, consider using a getter method.
interface IFoo {
readonly length: number;
}
class Foo implements IFoo {
private _length: number;
get length(): number {
return this._length;
}
change(length: number) {
this._length = length;
}
}
There is no functional difference between a variable with type Readonly<T> and a method that returns a variable with type Readonly<T>, except for the additional step of using a method.
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