What syntax is for (var i = 0, item; item = a[i++];)

Question

In the Re-Introduction to Javascript, the syntax

for (var i = 0, item; item = a[i++];)

is explained as the middle "item" being a conditional test for truthtiness/falsiness. However, I had assumed the syntax to be (start; condition test; control factor) with semi-colons between each segment. Here, the syntax is unfamiliar to me in the form (start, condition test; control factor;) with a comma in the middle and semicolon at the very end. Is it equivalent to

for (var i = 0; item; item = a[i++])

?

If so, why write it using comma and semicolon at the end?

Answer 1

In that expression, we have

• initialization = var i = 0, item -- this declares two variables, and assigns one of them.
• condition = item = a[i++] -- this performs an assignment, and tests the result of the assignment
• control factor = nothing -- the increment of i was done as part of the condition, so nothing is needed here

A for-loop is essentially equivalent to the following:

initialization;
while (condition) {
    body;
    control factor;
}

So when we substitute from your loop, we get:

var i = 0, item;
while (item = a[i++]) {
    // body that you didn't show
}

An assignment's value is the value that was assigned, so the condition is whether a[i] was truthy. No control factor is needed because a[i++] returns the value of a[i] and also increments i at the same time.

A more typical way to write this loop would be:

for (var i = 0; a[i]; i++) {
    var item = a[i];
    // body that you didn't show
}

The author was just showing how you can combine many pieces of this.

Answer 2

The format hasn't changed. It's simply declaring the var item. So, it is declaring two variables in the start section. The truthiness test is item = a[i++]; and the control factor is nothing.

for (;;) statement

Is a valid for statement. You don't HAVE to put anything in any of the sections.

item = a[i++] evaluates to true so long as item evaluates to true. The truthiness is done on the left-hand side of the assignment. That will depend on it's data type, but for an int, this could be any value besides 0 for example.