TypeScript does not accept union types in compatible overloads

Question

TypeScript does not accept union types in compatible overloads

When it comes to the Date object, you can create an instance using either a number or a string:

new Date("...");
new Date(123456);

Interestingly, while both number and string types are accepted, the union type string|number is not:

if (typeof value === "string" || typeof value === "number") {
    object = new Date(value); // Error.
}

Argument of type 'string | number' is not assignable to parameter of type 'string'. Type 'number' is not assignable to type 'string'.

This raises the question - what is the reasoning behind this behavior? It seems unnecessary to use an if...else chain when both accepted types should work. Why restrict usage in this case?

typescript

Answer 1

Answer №1

The TypeScript team initially decided against implementing union types with multiple overloads, as evidenced in their discussion on GitHub (Check it out here). Their stance was that it's preferable to avoid creating a series of overloads when equivalent representation can be achieved through union types. However, the current definition of DateConstructor in lib.d.ts includes multiple overloads:

interface DateConstructor {
    new (): Date;
    new (value: number): Date;
    new (value: string): Date;
    // ... etc...
}

To address this issue, one option is to assert one of the types to make it work:

if (typeof value === "string" || typeof value === "number") {
    object = new Date(value as string);
}

Alternatively, a new signature with a union type for DateConstructor could be created:

interface DateConstructor {
    new (value: number | string): Date;
}

This approach aligns more closely with what should ideally be done in lib.d.ts...

Answer 2

The TypeScript team initially decided against implementing union types with multiple overloads, as evidenced in their discussion on GitHub (Check it out here). Their stance was that it's preferable to avoid creating a series of overloads when equivalent representation can be achieved through union types. However, the current definition of DateConstructor in lib.d.ts includes multiple overloads:

interface DateConstructor {
    new (): Date;
    new (value: number): Date;
    new (value: string): Date;
    // ... etc...
}

To address this issue, one option is to assert one of the types to make it work:

if (typeof value === "string" || typeof value === "number") {
    object = new Date(value as string);
}

Alternatively, a new signature with a union type for DateConstructor could be created:

interface DateConstructor {
    new (value: number | string): Date;
}

This approach aligns more closely with what should ideally be done in lib.d.ts...

Answer 3

Answer №2

My suggestion would be to utilize the instanceof operator, although I am not completely certain about this approach. Furthermore, since you are checking for both types within a single condition, the code block still treats the value as the union type number|string. If you refer to the DateConstructor interface in lib.d.ts, you will see that there is no overload supporting this union type; instead, there are overloads for either a string or a number. Therefore, it is necessary to test each type separately.

Answer 4

My suggestion would be to utilize the instanceof operator, although I am not completely certain about this approach. Furthermore, since you are checking for both types within a single condition, the code block still treats the value as the union type number|string. If you refer to the DateConstructor interface in lib.d.ts, you will see that there is no overload supporting this union type; instead, there are overloads for either a string or a number. Therefore, it is necessary to test each type separately.

TypeScript does not accept union types in compatible overloads

Answer №1

Answer №2

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