How to mock and test Intl.DateTimeFormat in Jest

Intl.DateTimeFormat provides powerful functionalities to format dates.

This article demonstrates how to mock and test Intl.DateTimeFormat with Jest.

Example

Consider we have the following function that does format a given date into parts:

export const formatDateTimeToParts = (
  date,
  locale = "en-US",
  options = {
    month: "short",
    day: "numeric"
  }
) => {
  const dateTimeFormat = new Intl.DateTimeFormat(locale, options);
  const parts = dateTimeFormat.formatToParts(date);
  const dateTimeParts = {};
  parts.forEach((part) => {
    dateTimeParts[part.type] = part.value;
  });

  return dateTimeParts;
};

To use the above function, we can call it and pass at least the required parameter which is the first argument date, other parameters locale, and options are optional.

const formattedDate = formatDateTimeToParts(new Date());
console.log(JSON.stringify(formattedDate));

// Output:
// {"month":"Oct","literal":" ","day":"26"}

---

How to mock and test Intl.DateTimeFormat in Jest

We need to mock the implementation of Intl.DateTimeFormat by spying on any call to Intl.DateTimeFormat function, then we are going to return our mocked implementation.

In other words, we will add a listener to the DateTimeFormat calls, and once it’s called we will return our mocked implementation, as shown below:

const { DateTimeFormat } = Intl;
jest.spyOn(Intl, "DateTimeFormat")
  .mockImplementation(
   (locale, options) => new DateTimeFormat(locale, options)
  );

Then we can add our test implementation to verify our implementation:

  test("formatDateTimeToParts", () => {
    // Create a new data - note the month is zero indexed
    // 2022-08-15
    const currentDate = new Date(2022, 7, 15);
    const result = formatDateTimeToParts(currentDate, "en-US", {
      day: "2-digit",
      month: "long",
      year: "numeric"
    });

    expect(result).toStrictEqual({
      month: "August",
      literal: ", ",
      day: "15",
      year: "2022"
    });
  });

In the above test suite, we have created a new date object new Date(2022, 7, 15), Then we passed it along with the locale and format options to the formatDateTimeToParts function and we did verify the output of the function results.

---

Source code

---

Photo from Unsplash