Is there a way to consolidate the functions getStringOrFail and getNumberOrFail into a single function that handles different argument types?
export const getOrFail = <T>(value: T | null, message: string): T => {
if (value !== null) return value;
throw new Error(message);
}
I attempted using generics but encountered the issue of being able to return null. How can I refactor this code to achieve my goal?
Consider using a general approach, with the caveat that T must be a type that extends {}.
export function getOrFail<T extends {}>(value: T | null | undefined, message: string = "getNumberOrFail error"): T {
if (value != null) return value;
throw new Error(message);
}
{} indicates inclusion of all types except for null or undefined.
Further details can be found here.
To accomplish this, you need to define a generic parameter that excludes the possibility of being null:
export const getOrFail = <T extends string | number>(value: T | null, message: string = 'getOrFail error'): T => {
if (value !== null) return value
throw new Error(message);
}
In line with @jcalz's suggestion, for a more versatile approach that allows everything but null, you can use {} | undefined | void instead of string | number:
export const getOrFail = <T extends {} | undefined | void>(value: T | null, message: string = 'getOrFail error'): T => {
if (value !== null) return value
throw new Error(message);
}
Just wanted to share another method for the sake of completeness (not saying it's better than the rest), utilizing a conditional return type that utilizes Exclude to filter out null:
export const getOrFail = <T>(
value: T | null,
message: string = "getOrFail error"
) => {
if (value !== null) return value as Exclude<T, null>;
throw new Error(message);
};
You can test its behavior with these examples:
const bool = getOrFail(true); // boolean
const nevr = getOrFail(null); // never
const undef = getOrFail(undefined); // undefined
const strng = getOrFail(Math.random() < 0.5 ? "hello" : null); // string
The last example takes a parameter of type string | null and returns a type of
Exclude<string | null, null>string.
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