Javascript for loop with object instead of counter?

Question

Looking at this piece of code:

for (var i = 0, f; f = families[i]; i++) {
}

I haven't actually seen a loop like this before and I want to be sure I understand it correctly.
Am I correct in assuming that if families.length == 2 that the 2nd part of the for line would return false on f = families[2]?

I would have thought it would need to be something like f == families[2] in order to return false.

I'm intrigued, I assume that the loop won't stop till a certain criteria is met, not exactly when you process all the entries. — leopic
– leopic, Commented Sep 17, 2011 at 0:49

Raynos · Accepted Answer · 2011-09-17 01:09:43Z

11

f = families[i] is an expression that returns the value of families[i]. (It also has the side-effect of assigning that value to f)

If families.length === 2 then families[2] === undefined thus the expression returns undefined which is falsey and breaks the loop.

For more hacking fun you can turn

for (var i = 0, f; f = families[i]; i++) {
  // body
}

into

for (var i = 0, f; f = families[i++]; /* body */);

You may have to string replace ; with , and string replace i with i-1. You also just murdered readability.

It should also be pointed out that the for loop is silly for readability.

Object.keys(families).forEach(function(key) {
  var family = families[key];
  /* body */
});

Is significantly more readable.

edited Sep 17, 2011 at 1:09

answered Sep 17, 2011 at 0:49

Raynos

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5 Comments

Jared Farrish Over a year ago

You also just murdered readability. Heh heh. Colonel Mustard in the observatory with the keyboard.

Funka Over a year ago

Indeed, but also per LEOPiC's comment, it's also possible that a falsey value somewhere in the middle of the array would cause the loop to break, not always necessarily when you hit the end...

Raynos Over a year ago

@Funka since when is a family falsey ;)

user166390 Over a year ago

The last example is not semantically the same; families could have additional properties attached (or not be a "real array"). But otherwise, +1.

Raynos Over a year ago

@pst any competent programmer would make those additional properties non enumerable ;)

evan · Accepted Answer · 2011-09-17 01:04:47Z

0

This looks like kind of a silly way of doing

for(var i in families) {
    if (families.hasOwnProperty(i)) {
        // do whatever you want with families[i]
        console.log(families[i]);
    }
}

answered Sep 17, 2011 at 1:04

evan

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2 Comments

James P. Wright Over a year ago

This method doesn't make sense to me. Why use this versus a regular for loop? To me, the loop I posted is essentially taking out a line from a regular for loop where I would instantly say something like var family = families[i];

user166390 Over a year ago

This is different ... families could have properties that are not 0..n, even if "own".

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