Formatting currencies is a common problem. Let's say you are working on a website that supports multiple language, and is active in multiple countries. One of the problems that you will face is formatting an amount with a currency in a string that contains text. Example:

En: This will be $7.99.
Fr: This will be 7,99$.

Valid solution

One way of solving this problem is to have 2 distinct processes and call them one by one:

const amount: number = 7.99;
const formattedAmount = toCurrency(amount);
t("This will be {{ amount }}", { amount: formattedAmount });

This solution valid, but it could be nice to delegate this boilerplate to i18next.

i18next solution

Interpolation in i18next accepts a format function that can be implemented to handle formatting.

In your configuration:

interpolation: {
  escapeValue: false,
  // Add this line
  format: (value, format) => (format === "currency" ? toCurrency(value) : value),
},

Then, in your code:

t("This will be {{ amount, currency }}.", { amount: 12 })

This should format the currency properly.