Euler totient function in JavaScript [closed]

Well, first off, whitespace:

var phi=function(n){var count=" "  //an empty string
for(var i =1;i<=n;i++){
if(for(var j=1;j<=i;j++){
i%j===0;n%j===0;count += j.str;})   //appended the j as string into count
{return count.length;}}
};

has far to little. I reformatted and made this (put in a couple of semicolons, too):

var phi = function (n) {
    var count = " ";  //an empty string
    for (var i = 1; i <= n; i++) {
        if (for (var j = 1; j <= i; j++) {
            i % j === 0;
            n % j === 0;
            count += j.str; //appended the j as string into count
        }) {
            return count.length;
        }
    }
};

" " is not an empty string. "" is.

Now, why the heck do you have a for in an if? Try rewriting it without one. Array#all ought to help you with that. Once I get a chance I'll try my hand at it.

.str isn't needed. Just write count += j.

I can't comment on your algorithm because I can't tell what it is. This is why we write readable code. Don't use stupid hacks that make your program .03% faster that make you use stupid, unreadable tricks like, say, putting a for in an if. Prioritize readability until you absolutely have to do otherwise.