Question

for (var part; parts.length && (part = parts.shift());) {}

I thought a for loop can only be like this:

for (var part = 0; i < that.length; part++) {}

So what does that mean?

Answer 1

Reading this will probably explain a lot.

for (initial-expression; condition; final-expression) {
    // Block of code.
}

The initial expression is run once, before the loop starts.

The condition is checked for truth, and if true, the the final expression is run.

The final expression is run (along with the block of code) repeatedly until the condition is false.

You can put anything in any of these positions.

You can even do for(;;) { } which is known as the forever loop.

You can also do a purely conditional for loop.

for (prop in object) is the most common form of that.