I've e across a rather confusing statement in some JavaScript:

if (n = "value", a==b) {...

I take it that this assigns the value n first and then performs a parison (a==b) to determine whether to proceed with the if statement. But why? Is there any advantage to doing this over say...

n = "value";
if (a==b) {...

or...

if (a==b) {n = "value"; ...

I've e across a rather confusing statement in some JavaScript:

if (n = "value", a==b) {...

I take it that this assigns the value n first and then performs a parison (a==b) to determine whether to proceed with the if statement. But why? Is there any advantage to doing this over say...

n = "value";
if (a==b) {...

or...

if (a==b) {n = "value"; ...

• javascript

• 1 Your third snippet is not the same as two previous snippets. In two first snippets n is set in any case, in the third snippet n will be set only if if passes. – Teemu Commented Feb 5, 2014 at 17:36
• Hmm. That's true. What I was getting at, I suppose, is whether the scope of n is restricted in anyway by delcaring it in the if function (succinctly answered by Danilo below). In theory if n was restricted to the scope of the if statement then it could be available to the final parison, so even if the test fails n gets its value for the scope of if. – user1945782 Commented Feb 5, 2014 at 17:43
• @Question participants: (I know it's bad form on SO :-O ) but many thanks to you all for helping me understand this. I have actually used this now and found it makes the code more readable (in my current case) than less. – user1945782 Commented Feb 6, 2014 at 11:23

3 Answers 3

In JavaScript, whenever you put more than one expression inside a pair of brackets, they are evaluated as the last expression, like in the example below:

var a = (1, 2);
var b = a + 1;    // b = 2 + 1 = 3

So, in your case, the interpreter executes the attribution n = "value" and then parses the if taking a == b as condition. It's the same as:

n = "value";
if (a == b) {
    // ...
}

This article explains this behaviour.

EDIT

However, this does not limit n to the if's scope. This same thing happens to var declarations in for loops:

for (var i = 0; i < 10; i++) {
    // Do stuff...
}
console.log(i);    // Logs 10

EDIT 2

As Ethan Brown mentioned, is also good to tell about variable hoisting, which is basically the fact that, in JavaScript, values can be assigned to variables before declaring them. The following code shows this behaviour and was extracted from this MDN article:

bla = 2
var bla;

// The above code is valid, since
// it's implicitly understood as:

var bla;
bla = 2;

The same occurs with functions:

foo();
function foo() {
    console.log('bar');
}

Your assessment of the meaning is correct. There is no advantage other than pactness. Many would consider this poor practice, but it doesn't bother me.

Where it gets tricky is when you start calling functions that have side effects...then you can do some really weird stuff.

You're right - it's just a really confusing way to phrase assigning a variable and then running an if statement. It's valid code, but equivalent to the less puzzling version, so this is most likely just a case of someone being too clever.