TypeScript - function overloading

In this article, we would like to show how to overload function in TypeScript.

TypeScript supports function overloading syntax that lets to deflate multiple functions with the same name, different arguments and return types. It is required just to provide some functions declarations and one function definition.

Note: go to this article to know how to overload method in class.

Quick solution:

function someMethod(): ResultType;                          // <----- declaration
function someMethod(a: ArgType1): ResultType;               // <----- declaration
function someMethod(a: ArgType2, b: ArgType3): ResultType;  // <----- declaration
// ...

function someMethod(a?: any, b?: any): ResultType {         // <----- definition
    // ...                                                  // <----- definition
}                                                           // <----- definition

// ...

Practical example

function print(a: string): void;               // <----- declaration
function print(a: string, b: string): void;    // <----- declaration

function print(a: string, b?: string): void {  // <----- definition
    if (b) {                                   // <----- definition
        console.log(a + ' ' + b);              // <----- definition
    } else {                                   // <----- definition
        console.log(a);                        // <----- definition
    }                                          // <----- definition
}                                              // <----- definition

print('Hi!');
print('Hi', 'John!');

Output:

Hi!
Hi John!

Alternative solution

type Print = {                                           // <----- declaration
    (a: string): void;                                   // <----- declaration
    (a: string, b: string): void;                        // <----- declaration
};                                                       // <----- declaration

const print: Print = (a: string, b?: string): void => {  // <----- definition
    if (b) {                                             // <----- definition
        console.log(a + ' ' + b);                        // <----- definition
    } else {                                             // <----- definition
        console.log(a);                                  // <----- definition
    }                                                    // <----- definition
}                                                        // <----- definition

print('Hi!');
print('Hi', 'John!');

See also

1. TypeScript - class methods overloading