it-know-how/typescript/bits/13-optional-params-and-default-values.md

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

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

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.