TypeScript - Function Overloading

TypeScript provides the concept of function overloading. You can have multiple functions with the same name but different parameter types and return type. However, the number of parameters should be the same.

function add(a:string, b:string):string;

function add(a:number, b:number): number;

function add(a: any, b:any): any {
    return a + b;
}

add("Hello ", "Steve"); // returns "Hello Steve" 
add(10, 20); // returns 30 

In the above example, we have the same function add() with two function declarations and one function implementation. The first signature has two parameters of type string, whereas the second signature has two parameters of the type number.

The last function should have the function implementation. Since the return type can be either string or number as per the first two function declarations, we must use compatible parameters and return type as any in the function definition.

Function overloading with different number of parameters and types with same name is not supported.

function display(a:string, b:string):void //Compiler Error: Duplicate function implementation
{
    console.log(a + b);
}

function display(a:number): void //Compiler Error: Duplicate function implementation
{
    console.log(a);
}

Thus, in order to achieve function overloading, we must declare all the functions with possible signatures. Also, function implementation should have compatible types for all declarations.