Function overloading for optional arguments in Typescript

Overload functions to change signatures and provide optional arguments in Typescript.

We have previously seen an option to provide optional arguments to a function in Typescript. But, there is more to that to function overloads.

Building on our previous example -

function getSum(i: number, j?: number, k?: number): number {
  j = j || 0;
  k = k || 0;
  return i + j + k;
}

console.log(getSum(1));
// 1

In the example, we have simply made j and k optional arguments and made their value 0 if they are not provided. Although this is evident for someone who looks at the function definition, it is not so for a person looking at the declaration or the auto-complete when initializing this function.

Typescript provides a way to define function overloading during declaration so that we can make full use of the beautiful self-documenting feature.

Consider the same problem in the previous example written in a different way -

function getSum(i: number);
function getSum(i: number, j: number);
function getSum(i: number, j: number, k: number);
function getSum(i: number, j?: number, k?: number) {
  j = j || 0;
  k = k || 0;
  return i + j + k;
}

console.log(getSum(1)); // 1
console.log(getSum(1, 2)); // 3
console.log(getSum(1, 2, 3)); // 6

While the last function is the one that actually gets executed, the developer who consumes this function sees a function with 3 options - provide i alone, i & j, or all of i,j & k.

Function overloading does not cause any runtime overhead, but it is invaluable to further the self-documenting goals of your project.