Convert toUSD(amount) function to prototype function. How?

Question

I have the following function that works fine.

function toUSD(amount): string {
  // RETURN number in $0.00 format
  return new Intl.NumberFormat("en-US", {
    style: "currency",
    currency: "USD"
  }).format(amount);
};

I use it like this:

console.log(toUSD(123)); // output = $123.00

What would I need to change to use it like this?

console.log((123).toUSD()); // output = $123.00

Answer 1

Your function should not use input parameters to pass the value. You have to refer to the current number using this:

function toUSD(this: number): string {
  return new Intl.NumberFormat("en-US", {
    style: "currency",
    currency: "USD"
  }).format(this);
};

Then you can assign it to Number.prototype as also suggested by Ximaz' answer:

Number.prototype.toUSD = toUSD;

After that, you can call it like (123).toUSD() which outputs $123.00.