The "unexpected ++" error in jslint

Question

What is the best practice for that then?

Jslint explains that it "adds confusion". I don't see it really...

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Answer 1

The longstanding best practice: use i += 1 instead, following jslint's advice.

As for why it is a better practice than ++, according to Crockford:

The increment ++ and decrement -- operators make it possible to write in an extremely terse style. In languages such as C, they made it possible to write one-liners that: for (p = src, q = dest; !*p; p++, q++) *q = *p; Most of the buffer overrun bugs that created terrible security vulnerabilities were due to code like this. In my own practice, I observed that when I used ++ and --, my code tended to be too tight, too tricky, too cryptic. So, as a matter of discipline, I don’t use them any more.

Edit: Included comment from Nope as this answer continues to get views.

Answer 2

Just add /*jslint plusplus: true */ in front of your javascript file.

Answer 3

To avoid confusion, and possible problems when using minifiers, always wrap parens around the operator and its operand when used together with the same (+ or -).

var i = 0, j = 0;
alert(i++ +j);

This adds i and j (and increments i as a side effect) resulting in 0 being alerted.

But what is someone comes along and moves the space?

var i = 0, j = 0;
alert(i+ ++j);

Now this first increments j, and then adds i to the new value of j, resulting in 1 being alerted.

This could easily be solved by doing

var i = 0, j = 0;
alert((i++) +j); 

Now this cannot be mistaken.

Answer 4

Personally, I prefer to put statements such as i++ on a line by themselves. Including them as part of a larger statement can cause confusion for those who aren't sure what the line's supposed to be doing.

For example, instead of:

value = func(i++ * 3);

I would do this:

value = func(i * 3);
i++;

It also means people don't have to remember how i++ and ++i work, and removes the need to apply quite so many preference rules.

Answer 5

The real problem of the ++ operator is that it is an operator with side effects and thus it is totally opposed to the principle of functional programming.

The "functional" way to implement i++ would be i = i + 1 where you explicitly reassign the variable with no side effects and then use it.

The possibility of confusion is that ++ does two things by adding a value AND reassigning it to the variable.

Answer 6

JSLint friendly loop

for (i = 0; i < 10; i += 1) {
    //Do somthing
}

Answer 7

Please note that the ++ operator depends on position with respect to the prior/next variable and the newline / semicolon to determine order of operations.

var a = 1;
var b = a++;
console.log(b); // b = 1
console.log(a); // a = 2

var a = 1;
var b = ++a;
console.log(b); // b = 2
console.log(a); // a = 2

Answer 8

There is something called a pre-increment: ++i and a post-increment i++ and there is a difference:

var i = 9;
alert(++i); //-> alerts 10

var j = 9;
alert(j++); //-> alerts 9
alert(j);   //-> alerts 10 now, as expected

var k = 9;
alert((k++)); //-> still alerts 9 even with extra parentheses