Guard Clause in Async

egig.github.io | August 20th, 2018 | #javascript

I really like Guard Clause, it makes me easier to read coding flow. Ya, you might want to google it “Guard Clause”, is a pattern to write code. You’ll end up on this link: Replace Nested Conditional with Guard Clauses - Refactoring. If you don’t use it or even don’t know it yet, I suggest you to learn and use it and share to your team. You all will love it.

But, when it comes to javascript, its always be exhausted. Javascript is exhausting. I dive in to async world in javascript two years ago and still don’t know to use Guard Clause in it. Ya, this code won’t work whatsoever:

function getItem(type) {

    if(type == 1) {
        return request.get("/onetype", (err, results) => {
          if(results.length) {
            return null;
          }

          return results[0];
        })
    }

    if(type == 2) {
         return request.get("/another_type", (err, results) => {
          if(results.length) {
            return null;
          }

          return results[0];
        })
    }
   

    return null;
}

let item =  getItem(); // Won't return the desired item

So, how do we do that ? I don’t think we can do “Guard Clause” for async.

Lets take example for Martin Fowler website above, but in js.

function getPayAmount() {
  if (_isDead) return deadAmount();
  if (_isSeparated) return separatedAmount();
  if (_isRetired) return retiredAmount();
  return normalPayAmount();
};

If functions deadAmount, separatedAmount, retiredAmount and normalPayAmount are Promises, I used to do it like this.

function getPayAmount(callback) {
  if (_isDead) return deadAmount().then(callback);
  if (_isSeparated) return separatedAmount().then(callback);
  if (_isRetired) return retiredAmount().then(callback);
  return normalPayAmount().then(callback);
};

Ya but actually we can do it better like this.

function getPayAmount() {
  if (_isDead) return deadAmount();
  if (_isSeparated) return separatedAmount();
  if (_isRetired) return retiredAmount();
  return normalPayAmount();
}; 

getPayAmount().then((amount) => {
   // do whatever with amount
});

Or better better, we can use IIFE if getPayAmount doesn’t necessary to be reusable function.

(function getPayAmount() {
  if (_isDead) return deadAmount();
  if (_isSeparated) return separatedAmount();
  if (_isRetired) return retiredAmount();
  return normalPayAmount();
};)()
.then((amount) => {
   // do whatever with amount
})