author: Krasimir Tsonev

Hi there, I'm . Senior front-end engineer with over 13 years of experience. I write, speak and occasionally code stuff.

An alternative JavaScript for loop syntax

As javascript developers we all have to write a lot of for loops. Before a couple of months I saw an alternative syntax, which I really liked. It saves some time and it looks much nicer. However, one of my colleagues reported that this fancy syntax is not working as it should.

Let's say that I have an array of data representing users in a system. What I did before is:

var users = [
    { name: "A"},
    { name: "B"},
    { name: "C"},
    { name: "D"},
    { name: "E"}
];

There is one additional row var user = users[i];. Normally I feel more comfortable if I have user instead of users[i]. So, the new way:

for(var i=0; user=users[i]; i++) {
    // ...
}

I wrote a question in Stackoverflow and the guys there spotted the problem. This syntax will not work if some of the elements of the array is falsy. I.e. if I have:

var users = [
    { name: "A"},
    { name: "B"},
    false,
    { name: "D"},
    { name: "E"}
];

The loop will stop on the third element, because user will be false. We could of course solve this by checking the type of the variable:

for(var i=0; typeof (user=users[i]) !== "undefined"; i++) {
   // ...
}

Even this, is not perfect, because if something is undefined we will be in the same situation.

The conclusion is that we can use the second syntax only if we are 100% sure that our elements are truly.

If you enjoy this post, share it on Twitter, Facebook or LinkedIn. Or maybe comment below: