On: Bad TypeScript Habits

[bengalister]

About #2 Wasn't so sure about typescript so I tested it to confirm that it worked. But in Python3, the default arguments are evaluated at module time loading if I am not mistaken. The following will display roughly the same timestamp

  from datetime import datetime
  import time

  print(datetime.utcnow())
  def show_ts(d=datetime.utcnow()):
      print(d)

  time.sleep(5)
  show_ts()

where as in TS, the 2nd one will be roughly 5s older:

  const now = Date.now();
  console.log(now);

  function logTs(d=Date.now()) {
     console.log(d);
  }

  setTimeout(()=> {
     logTs();
  }, 5000);

[horsawlarway → bengalister]

That's because default params in JS are essentially syntactic sugar ex -

  function example(a = 1) {
  ...
  }

behaves the same as

  function example(a) {
  a = (typeof a !== 'undefined') ?  a : 1
  ...
  }

[sorahn → horsawlarway]

Which also means the suggestion to use ?? has a different result then using a default param. ?? will catch both null and undefined, where as a default parameter will only trigger on undefined.

[nawgz → sorahn]

?? catches any falsy value, so it is quite semantically different than default arguments, and also quite different from ||

[Dylan16807 → nawgz]

Did you write that backwards? || checks for falsy, ?? checks for null/undefined

[horsawlarway → sorahn]

Yup, you're absolutely correct. Null is considered a valid value for a default param.

  function test(a = 1) {
    console.log(a)
  }

  test() // -> 1 is printed
  test(null) // -> null is printed
  test(undefined) // -> 1 is printed

[Dylan16807 → horsawlarway]

Not because.

You could have sugar evaluated once, sugar evaluated each time, not-sugar evaluated once, or not-sugar evaluated each time.

[tempodox → bengalister]

Events that happen at a later time are called “younger”, not “older”. If you were born at a later date than me, you're younger than me.

[bengalister → tempodox]

Yes, the logging event is younger, the timestamp displayed is of greater value. Considering the epoch time as the date of birth the displayed timestamp is older. Probably a translation issue from my native language.