How does one make an optional closure in swift?

You should enclose the optional closure in parentheses. This will properly scope the ? operator.

func then(onFulfilled: ()->(), onReject: (()->())?){       
    if let callableRjector = onReject {
      // do stuff! 
    }
 }

To make the code even shorter we can use nil as default value for onReject parameter and optional chaining ?() when calling it:

func then(onFulfilled: ()->(), onReject: (()->())? = nil) {
  onReject?()
}

This way we can omit onReject parameter when we call then function.

then({ /* on fulfilled */ })

We can also use trailing closure syntax to pass onReject parameter into then function:

then({ /* on fulfilled */ }) {
  // ... on reject
}

Here is a blog post about it.

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