Regarding JavaScript for() loop voodoo

Question

I was for quite some time under the impression that a for loop could exist solely in the following format:

for (INITIALIZER; STOP CONDITION         


        

Answer 1

The generalized format of a for loop (not a for-in loop) is

for ( EXPRESSION_1 ; EXPRESSION_2 ; EXPRESSION_3 ) STATEMENT

The first EXPRESSION_1 is usually used to initialize the loop variable, EXPRESSION_2 is the looping condition, and EXPRESSION_3 is usually an increment or decrement operation, but there are no rules that say they have to behave like that. It's equivalent to the following while loop:

EXPRESSION_1;
while (EXPRESSION_2) {
    STATEMENT
    EXPRESSION_3;
}

The commas are just an operator that combines two expressions into a single expression, whose value is the second sub-expression. They are used in the for loop because each part (separated by semicolons) needs to be a single expression, not multiple statements. There's really no reason (except maybe to save some space in the file) to write a for loop like that since this is equivalent:

shuffle = function(o) {
    var j, x;
    for (var i = o.length; i > 0; i--) {
        j = parseInt(Math.random() * i);
        x = o[i - 1];
        o[i - 1] = o[j];
        o[j] = x;
    }
    return o;
};