4

Snippet 1: 

if ( x ) { 
    foo();
}

Snippet 2: 

x ? foo() : 0;

What are the differences between those two snippets?

Edit: Corrected the syntax error.

Update: btw, it seems that there is an even shorter notation: 

x && foo();
Šime Vidas
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3 Answers3

8

Snippet #2 is an invalid ECMAScript expression since it lacks the required : blah in order to make it a ternary.

EDIT: There isn't really a difference between the two snippets, they both invoke foo if x is truthy. If x is falsy (undefined/false/empty string/0/) then first snippet wouldn't evaluate to anything, latter snippet would evaluate to 0 but that 0 really has no impact on the script.

meder omuraliev
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  • Also, the block syntax does not evaluate to any value (it's not an expression). The ternary operator does evaluate to a value. – cdhowie Dec 02 '10 at 22:31
  • @cdhowie Yea, but the ternary expression is not assigned to anything in my code, so this is not a difference in this case. – Šime Vidas Dec 02 '10 at 22:36
  • If we had `x ? foo() : bar()` and the functions didn't return anything. Would it be ECMA valid because it's actually value `undefined`? – Martin Algesten Dec 02 '10 at 22:36
  • @Martin: Yes. Expressions can be used as statements. And any function can return `undefined` without error. It's when you try to *use* the value `undefined` that you crash. – cdhowie Dec 02 '10 at 22:40
  • @meder OK, I figured it out. Thanks. (I thought there might be a slight distinction between those two, but I guess there isn't.) – Šime Vidas Dec 02 '10 at 22:44
  • Ok do it works as expected, but do I interpret the answer as "yes that is good ECMA script"? – Martin Algesten Dec 02 '10 at 22:46
  • @Martin It's a shortcut notation. As long as your right-hand sides are just expressions, you should be fine. – Šime Vidas Dec 02 '10 at 22:52
3

I think you meant to ask the following

// Why should I use
if (x) {
  do();
}

// instead of 
x && do();

or 

// Why should I use
if (x) {
  do(); 
} else {
  dont();
}

//instead of 
x ? do() : dont()

As they are written, there is no difference in the output. I don't like using the ternary or the && operator in these cases because of the semantics of using the ternary and &&. A ternary returns a value that you're not using, as does the && expression. Therefore, I'd rather use the way the language intends operators to be used.

However, I love using && for null checks

var val = obj && obj.getValue();

Note that in this case, we are using the return value of the expression

Ruan Mendes
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1

Generally, if is a statement (e.g. you can't assign it to a variable) and the ternary operator ?: is part of an expression (yields a value which can be assigned to variables).

Also, the if block can contain statements while the components of ?: can only contain expressions.

In the example you give, there is no difference, since you don't use the result of the ?:. Both snippets evaluate foo() only if x is a true value.

Amnon
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