Comma in an if statement

Question

I've come 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 comparison (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"; ...

Answer 1

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');
}