Function Overloading in TypeScript

Function overloading in TypeScript allows you to define multiple signatures for a function, each with a different set of parameter types or a different number of parameters. This feature enables you to create more flexible and versatile functions that can handle various input scenarios while maintaining type safety.

Understanding Function Overloading

In TypeScript, you can overload functions by defining multiple function signatures followed by a single implementation. The implementation must be able to handle all defined signatures.


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

console.log(add(5, 10)); // Output: 15
console.log(add("Hello, ", "TypeScript!")); // Output: Hello, TypeScript!

Defining Multiple Function Signatures

When using function overloading, you define multiple function signatures before the actual implementation. These signatures specify the different combinations of parameter types that the function can accept.


function getLength(value: string): number;
function getLength(value: any[]): number;
function getLength(value: any): number {
  return value.length;
}

console.log(getLength("Hello")); // Output: 5
console.log(getLength([1, 2, 3, 4])); // Output: 4

Overloading with Different Parameter Counts

Function overloading can also be used to define functions that accept a different number of parameters. In this case, the implementation must handle the varying number of parameters appropriately.


function greet(name: string): string;
function greet(firstName: string, lastName: string): string;
function greet(firstName: string, lastName?: string): string {
  if (lastName) {
    return `Hello, ${firstName} ${lastName}!`;
  } else {
    return `Hello, ${firstName}!`;
  }
}

console.log(greet("Alice")); // Output: Hello, Alice!
console.log(greet("Alice", "Johnson")); // Output: Hello, Alice Johnson!

Use Cases for Function Overloading

Function overloading is particularly useful in the following scenarios:

Creating functions that accept different types of parameters
Handling different numbers of parameters with a single function
Improving code readability and maintainability by grouping related functionality

Conclusion

Function overloading in TypeScript provides a powerful way to create flexible and versatile functions that can handle various input scenarios. By defining multiple function signatures and a single implementation, you can maintain type safety while accommodating different parameter types and counts. This feature helps improve code readability and maintainability, making your TypeScript applications more robust and easier to understand.

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