What is preventing me from invoking an overloaded function using this approach?

Question

What is preventing me from invoking an overloaded function using this approach?

Consider this scenario involving function overloading:

function selectItem(x: {type: string; id: number; }[]): number;
function selectItem(x: number): {type: string; id: number; };
function selectItem(x): any {
      /* ... */
}

The documentation explains:

It is important to note that the function selectItem(x): any section is not considered part of the overload list, so there are only two defined overloads: one for an object parameter and one for a numerical parameter. Invoking selectItem with any other types as parameters will result in an error being thrown.

This means that calling selectItem("test"); would trigger an error.

If selectItem(x) does not represent a valid function signature, why include it at all? What advantages does its existence offer? Is there a specific use case that I might be overlooking?

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typescript overloading

Answer 1

Answer №1

Signatures are used to specify the different options available for calling a function.

The third part contains the actual function definition, which takes in one argument and returns any type of value.
Within this section, the logic of the function is implemented. It checks the types of the input received and returns the appropriate value based on the parameters passed.

For instance:

function a(str: string): string;
function a(num: number): number;
function a(value: any): any {
    if (typeof value === "string") {
        return (parseInt(value) * 2).toString();
    } else {
        return value * 2;
    }
}

a("3"); // should yield "6"
a(3); // should yield 6
a(true); // error

The presence of the third part is necessary because without it, one would have to use either of the other signatures as the actual function definition.
If the first signature is chosen, then the parameter must be a string rather than a number. If the second one is selected, then the parameter must be a number instead of a string.
Hence, the purpose of the third part is to determine the actual parameters that were provided and execute the corresponding logic accordingly.

It is also possible to have varying numbers of properties:

function b(a: number): number;
function b(a: number, b: number): number;
function b(...args: number[]): any {
    if (args.length == 1) {
        ...
    } else {
        ...
    }
}

This scenario typically involves using different signatures when dealing with the same number of parameters but of different types. In such cases, you can simply write:

function a(value: string | number): any {
    if (typeof value === "string") {
        return (parseInt(value) * 2).toString();
    } else {
        return value * 2;
    }
}

Answer 2

Signatures are used to specify the different options available for calling a function.

The third part contains the actual function definition, which takes in one argument and returns any type of value.
Within this section, the logic of the function is implemented. It checks the types of the input received and returns the appropriate value based on the parameters passed.

For instance:

function a(str: string): string;
function a(num: number): number;
function a(value: any): any {
    if (typeof value === "string") {
        return (parseInt(value) * 2).toString();
    } else {
        return value * 2;
    }
}

a("3"); // should yield "6"
a(3); // should yield 6
a(true); // error

The presence of the third part is necessary because without it, one would have to use either of the other signatures as the actual function definition.
If the first signature is chosen, then the parameter must be a string rather than a number. If the second one is selected, then the parameter must be a number instead of a string.
Hence, the purpose of the third part is to determine the actual parameters that were provided and execute the corresponding logic accordingly.

It is also possible to have varying numbers of properties:

function b(a: number): number;
function b(a: number, b: number): number;
function b(...args: number[]): any {
    if (args.length == 1) {
        ...
    } else {
        ...
    }
}

This scenario typically involves using different signatures when dealing with the same number of parameters but of different types. In such cases, you can simply write:

function a(value: string | number): any {
    if (typeof value === "string") {
        return (parseInt(value) * 2).toString();
    } else {
        return value * 2;
    }
}

What is preventing me from invoking an overloaded function using this approach?

Answer №1

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