Weird looking Javascript for loop

Question

I have never seen a JavaScript loop such as this for( ; i-- ; ), used in the code:

uid: function (len) {
    var str = '';
    var src = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var src_len = src.length;
    var i = len;

    for (; i--;) {
        str += src.charAt(this.ran_no(0, src_len - 1));
    }

    return str;
}

I understand the behavior, but I would like it if somebody could share some insights about this type of for loop.

Answer 1

This is a syntax of the for-loop construction:

for ([initialization]; [condition]; [final-expression])
     statement

In your case for (; i--;) {:

• no variables are initialized, because var i = len; inintialized earlier, so it's not needed.
• condition part will be truthy until i becomes 0 then loop will terminate. i-- is executed on before each iteration, and due to -- operator it will eventually become 0, so it's falsy, and loop will stop.
• since i is decremented in condition part of the loop, final-expression is not needed too. Another way to put it: since i is not used inside the loop, it does not matter whether we decrement it before each loop iteration or after each loop iteration.

That being said, it's better to avoid writing loops like above, as it's pretty confusing and hard to read. Prefer traditional for-loops notation.

Answer 2

From MDN - for - Optional for expressions:

All three expressions in the head of the for loop are optional.

You don't have to specify all three expressions in for loops. For example, for (;;) is a common wa of writing infinite loop.

In your case, while(i--) would have done the same, there is no good reason to write for (; i--;).

I'd also note that for(var i=len;i>=0;i--) is more robust - it protects you from the case len is negative.

Answer 3

This could be rewritten to

uid: function (len) {
    var str = '';
    var src = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var src_len = src.length;
    var i = len;

    while (i >= 0) {
        str += src.charAt(this.ran_no(0, src_len - 1));
        i = i - 1;
    }

    return str;
}

The for statement creates a loop that consists of three optional expressions.

Javascript consider 0 == false that's why in the case you presented the loop will run until the i variable became zero. It will loop as many times as the src string size.

Note: i-- uses the variable value then decrements it. Take a look at the following situation:

for(;i--;) { // i = length in the condition
     // i === length - 1 here. It will not overflow the array
}

for(;i--;) { // i = 1
     // i === 0 here. It will be the last loop
}

for(;i--;) { // i == 0 == false
     // Not executed
}  

Answer 4

There is nothing wrong.

for(`INIT`;`TEST`;`ACTION`)
{
     `WORK`;
}

The INIT (initialization) can be done outside the loop.

var i=0;
for(;i<=100;i++)
//do something

The TEST part yield a result that is either true or false. Now in this case value of i is tested. Until it becomes zero this works.

The ACTION part is generally used to change the loop variable. But you can leave it also or probably add it to the TEST section like it is done here.

Look this examples are going to clear your idea

var i=0;
for( i++; i++; i+=j); //No body

var i=0;
for(;;); //an infinite loop

var i;
for(i=-4;i;i++);//

Even sometimes WORK is placed in ACTION.

Example:

factorial of x

 for(var i=1,ans=1;i<=x;ans=ans*(i++));

---

Which can be written this way also-


 var ans=1;
for(var i=1;i<=x;i++)
   ans=ans*i;

NOTE: You can write whichever way you want. It doesn't matter as long as you have written the code properly. Get used to this kind of form you will see them a lot.

Though, it is better to write sometimes in compact form , but remember you should keep the readability.

Answer 5

That's just for loop. Before the first semicolon the variable used in the loop is usually declared. But in this case the variable, the variable used in the loop is declared earlier:

var i = len;
   ^^^
for (; i--;) {...

So there is no need to redeclare.

After the first semicolon, there is a condition for the loop to run (i.e. i<6, i>3, etc). If condition returns false, or 0, the loop is exited. In this case, the loop will be broken out when i is 0. It happens eventually, because -- after i decrements it, and therefore there is no need for the expression, which is the place after the second semicolon is reserved for.

Answer 6

The first parameter of the for loop has already been defined, so the initial comma is there to delimit it's place in the parameter list.