Are braces necessary in one-line statements in JavaScript?

I once heard that leaving the curly braces in one-line statements could be harmful in JavaScript. I don't remember the reasoning anymore and a Google search did not help much.

Is there anything that makes it a good idea to surround all statements within curly braces in JavaScript?

I am asking, because everyone seems to do so.

No

But they are recommended. If you ever expand the statement you will need them.

This is perfectly valid

if (cond) 
    alert("Condition met!")
else
    alert("Condition not met!")

However it is highly recommended that you always use braces because if you (or someone else) ever expands the statement it will be required.

This same practice follows in all C syntax style languages with bracing. C, C++, Java, even PHP all support one line statement without braces. You have to realize that you are only saving two characters and with some people's bracing styles you aren't even saving a line. I prefer a full brace style (like follows) so it tends to be a bit longer. The tradeoff is met very well with the fact you have extremely clear code readability.

if (cond) 
{
    alert("Condition met!")
}
else
{
    alert("Condition not met!")
}

There's a readability aspect - in that when you have compound statements it can get very confusing. Indenting helps but doesn't mean anything to the compiler/interpreter.

var a;
var b;
var c;

//Indenting is clear
if (a===true)
  alert(a); //Only on IF
alert(b); //Always

//Indenting is bad
if (a===true)
  alert(a); //Only on IF
  alert(b); //Always but expected?

//Nested indenting is clear
if (a===true)
  if (b===true)
    alert(a); //Only on if-if
alert (b); //Always

//Nested indenting is misleading
if (a===true)
  if (b===true)
    alert(a); //Only on if-if
  alert (b); //Always but expected as part of first if?

//Compound line is misleading
//b will always alert, but suggests it's part of if
if (a===true) alert(a);alert(b); 
else alert(c); //Error, else isn't attached

And then there's an extensibility aspect:

//Problematic
if (a===true)
  alert(a);
  alert(b); //We're assuming this will happen with the if but it'll happen always
else       //This else is not connected to an if anymore - error
  alert(c);

//Obvious
if (a===true) {
  alert(a); //on if
  alert(b); //on if
} else {
  alert(c); //on !if
} 

The thinking goes that if you always have the brackets then you know to insert other statements inside that block.

The question asks about statements on one line. Yet, the many examples provided show reasons not to leave out braces based on multiple line statements. It is completely safe to not use brackets on one line, if that is the coding style you prefer.

For example, the question asks if this is ok:

 if (condition) statement;

It does not ask if this is ok:

 if (condition)
   statement;

I think leaving brackets out is preferable because it makes the code more readable with less superfluous syntax.

My coding style is to never use brackets unless the code is a block. And to never use multiple statements on a single line (separated by semicolons). I find this easy to read and clear and never have scoping issues on 'if' statements. As a result, using brackets on a single if condition statement would require 3 lines. Like this:

 if (condition) {
   statement;
 }

Using a one line if statement is preferable because it uses less vertical space and the code is more compact.

I wouldn’t force others to use this method, but it works for me and I could not disagree more with the examples provided on how leaving out brackets leads to coding/scoping errors.