Overload Functions in TypeScript

Function overloading in TypeScript allows you to have multiple functions with the same name but with a different signature (parameters and types). This means that the parameters have different types or the number of parameters is different for each function. The correct function to call is determined at runtime based on the arguments passed.

For example, consider a function that takes an argument and returns its square. If the argument is a number, you can calculate the square by multiplying the number by itself. However, if the argument is an array of numbers, you can calculate the square by squaring each element of the array.

function square(x: number): number;
function square(x: number[]): number[];
function square(x: number | number[]): number | number[] {
    if (typeof x === 'number') {
        return x * x;
    } else {
        return x.map(e => e * e);
    }
}

square(3); // => 9
square([3, 5, 7]) // => [9, 25, 49]

Here the first two lines describe the function signature with two different argument types, number and number[]. The actual function implementation begins on the third line, and its signature must be a union of number | number[]. The implementation uses the typeof operator to determine which branch of the implementation to use.

Another example would be a database query function that reads a user from the database based on an ID (number), username (string), or attributes (object).

function query(id: number): User;
function query(username: string): User;
function query(attributes: Record<string, any>): User;
function query(arg: number | string | Record<string, any>): User {
    let condition = '';

    if (typeof arg === 'number') {
        // Query the database using an id
        condition = `id: ${arg}`;
    } else if (typeof arg === 'string') {
        // Query the database using a username
        condition = `username: "${arg}"`;
    } else {
        // Query the database using attributes
        condition = JSON.stringify(arg);
    }

    return db.query(`SELECT * FROM User WHERE ${condition}`);
}

query(1); // => id: 1
query('johndoe'); // => username: johndoe
query({ firstName: 'John', lastName: 'Doe' }); // => attributes: {"firstName":"John","lastName":"Doe"}

In this example, the query function is overloaded three times, each time with a different signature. The first signature accepts a single parameter of type number, the second signature accepts a single parameter of type string, and the third signature accepts a single parameter of type Record<string, any>, which is a simple key-value map.

The examples I gave earlier show how you can use function overloading to process different types of input and give you an idea of how it can be useful in practice.