Check whether a value is defined (non-nullable), meaning it is neither `null` or `undefined`. This can be useful as a type guard, as for example, `[1, null].filter(Boolean)` does not always type-guard correctly.

The reason Final Form does this is so that pristine will be true if you start with an uninitialized form field (i.e. value === undefined), type into it (pristine is now false), and then empty the form field. In this case, pristine should return to true, but the value that the HTML DOM gives for that input is ''. If Final Form did not treat '' and undefined as the same, any field that was ever typed in would forever be dirty, no matter what the user did.

There are times where it is useful to know whether a value was passed to run or the result of a filter default. In particular, it is useful when nil is an acceptable value.

main > h = {key_with_nil_value: nil}
=> {:key_with_nil_value=>nil}

main > h[:key_with_nil_value]
=> nil

main > h[:missing_key]  # this would be undefined in JavaScript (a useful distinction) rather than null, but in Ruby it's indistinguishable from the case where a nil value was actually explicitly _supplied_ by the caller/user
=> nil

# so we have to check for "missingness" ("undefinedness"?) differently in Ruby

main > h.key?(:key_with_nil_value)
=> true

main > h.key?(:missing_key)
=> false

In TypeScript, we can also set a value that a parameter will be assigned if the user does not provide one, or if the user passes undefined in its place.

buildName(undefined, "Adams")

If a default-initialized parameter comes before a required parameter, users need to explicitly pass undefined to get the default initialized value.

There are many strings which I need to convert if not undefined. I could do as follows: { date1: date1 ? formatISODate(date1) : undefined, date2: date2 ? formatISODate(date2) : undefined ...