Vjeux

Reading An Open Letter to JavaScript Leaders Regarding Semicolons where Isaac Z. Schlueter explains his unorthodox coding style a line of code struck me.

if (!cb_ && typeof conf === "function") cb_ = conf , conf = {}

He was able to execute more than one statement in a if without the need of { }. I have recently been working on python scripts for http://db.mmo-champion.com/ and this discovery made me want to imitate pythonic indentation in Javascript.

The comma trick

You can use the , separator to chain statement. This group them into only one block of code. Therefore you can execute all of them without the need of { }. The rule is easy: put a , at the end of every line but a ; on the last line of the block.

if (test)
  first_action(), // Note the important ','
  second_action(); // Note the lack of ','
third_action();

For example, it is possible to write a little program that outputs the Fibonacci Numbers without the use of any { } and therefore imitate python indentation style with no ending }.

var curr = 0, next = 1, tmp;
for (var i = 0; i < 10; ++i)
  tmp = curr + next,
  curr = next,
  next = tmp,
  console.log('Fibo', i, '=', curr);
 
// ...
// Fibo 5 = 8
// Fibo 6 = 13
// Fibo 7 = 21
// Fibo 8 = 34
// ...

The issues

Sadly, the use of this is trick is extremely limited. You cannot use any of these keywords inside the "blocks": if, for, var.

for (var i = 0; i < 3; ++i)
  k = i * 10 + 1,
  if (k % 2 == 0)
    console.log(i);
// SyntaxError: Unexpected token if
 
for (var i = 0; i < 3; ++i)
  var k = 10,
  console.log(k);
// Firefox: SyntaxError: missing ; before statement
// Chrome: SyntaxError: Unexpected token .

Beginning with comma

If you don't fall into the use cases of these issues and you are a bit worried about the bugs resulting in the mix of the , and ;, you can start your lines with commas.

var k;
for (var i = 0; i < 10; ++i)
  , k = i * 10
  , console.log(i)
// SyntaxError: Unexpected token ,

But we need to add some empty statement before the first , so that it compiles. In python : is used but it doesn't parse in Javascript. We can use $ for example, it is a valid statement: it reads the variable and does nothing with it.

var $;
for (var i = 0; i < 10; ++i)$ // Use of $ instead of : in python
  , k = i * 10
  , console.log(k)
// 0
// 10
// ...

Debugging purpose

The main use of this trick I can see is for debugging purpose. If there is code executed in a test without { } and you want to log something when the program goes into this part of the code. Before you had to add { } and then remove them which is really annoying. Now it's easier!

for (test)
  doSomething();

// Before
for (test) {
  val = doSomething();
  console.log('Executed!', val);
}
 
// After
for (test)
  val = doSomething(),
  console.log('Executed!', val);

Conclusion

Using the comma trick to do { }-less indentation is far from viable. However this may still be useful for debugging and overall it is fun to try new coding styles!