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Why one uses async AND defer simultanously?

I see from time to time, people use async and defer simultanously for same (third-party) scripts. Why one does this? Aren't async and defer thought for completely different cases?

Solution

  • It's an old technique for when the async attribute wasn't well supported (IE supported defer until IE 10 but not async, but even then it only supported defer badly).

    Unfortunately it's now bad advice as it means defer is ignored as per MDN linked in fdomnn-m's comment:

    defer

    If the attribute is specified with the async attribute, the element will act as if only the async attribute is specified.

    And usually you want the more powerful defer as opposed to async.

    So people should stop doing this.