Looking for a way to inform TypeScript that the output of a function will always meet the type requirements of a variable, therefore avoiding any error messages?
Type 'string | Date' is not assignable to typeFor example:
const getStringOrDate(asString = true) => {
if (asString) {
return new Date().toLocaleDateString()
} else {
return new Date()
}
}
const today: Date = getStringOrDate(false);
^ Type 'string | Date' is not assignable to type Date
You have the flexibility to define a function multiple times with different types and assign diverse return types. To specify a return type, simply append it after the brackets. For instance, let fn = (): number => 1;
You can achieve this by defining the function like:
function getStringOrDate(asString?: true = true): string;
function getStringOrDate(asString?: false = true): Date;
function getStringOrDate(asString = true): Date | string {
return asString ? new Date().toLocaleString() : new Date();
}
Remember, this method is only applicable when using the traditional function keyword, not ES5 arrow functions.
By following this syntax, you can accurately set the return type to either a string or a Date object.
In addition to implementing multiple functions (which is usually recommended), another approach is using a conditional return type. Here's an example:
function getStringOrDate<T extends boolean = true>(
asString = true as T
): T extends true | undefined ? string : Date {
return (asString ? new Date().toLocaleDateString() : new Date()) as any;
}
The drawback of this technique is the requirement for casting, but in certain specific cases, implementations like these may be more idiomatic.
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