# Optional Parameters and Default Values in TypeScript

In TypeScript, a function parameter can be optional and can also have a default value.

## Example from your code

```ts
function buildWeeklyDownloadPeriods(
  dateRange?: DownloadDateRange,
  now = new Date(),
): string[] {
  // ...
}
```

## What this means

- `dateRange?`:
  - The `?` means this argument is optional.
  - The caller can omit it.
- `now = new Date()`:
  - If caller does not pass `now`, TypeScript uses `new Date()`.

## Python comparison

```py
def build_weekly_download_periods(date_range=None, now=None):
    if now is None:
        now = datetime.now()
```

TypeScript default parameters are cleaner because the default is declared directly in the signature.