Guard Clause in Async

20 Aug, 2018

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:

 1function getItem(type) {
 2
 3    if(type == 1) {
 4        return request.get("/onetype", (err, results) => {
 5          if(results.length) {
 6            return null;
 7          }
 8
 9          return results[0];
10        })
11    }
12
13    if(type == 2) {
14         return request.get("/another_type", (err, results) => {
15          if(results.length) {
16            return null;
17          }
18
19          return results[0];
20        })
21    }
22   
23
24    return null;
25}
26
27let 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.

1function getPayAmount() {
2  if (_isDead) return deadAmount();
3  if (_isSeparated) return separatedAmount();
4  if (_isRetired) return retiredAmount();
5  return normalPayAmount();
6};

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

1function getPayAmount(callback) {
2  if (_isDead) return deadAmount().then(callback);
3  if (_isSeparated) return separatedAmount().then(callback);
4  if (_isRetired) return retiredAmount().then(callback);
5  return normalPayAmount().then(callback);
6};

Ya but actually we can do it better like this.

 1function getPayAmount() {
 2  if (_isDead) return deadAmount();
 3  if (_isSeparated) return separatedAmount();
 4  if (_isRetired) return retiredAmount();
 5  return normalPayAmount();
 6}; 
 7
 8getPayAmount().then((amount) => {
 9   // do whatever with amount
10});

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

1(function getPayAmount() {
2  if (_isDead) return deadAmount();
3  if (_isSeparated) return separatedAmount();
4  if (_isRetired) return retiredAmount();
5  return normalPayAmount();
6};)()
7.then((amount) => {
8   // do whatever with amount
9})

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