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Fighting the uglyness of Array.push()

Fighting the uglyness of Array.push()

Say you are creating an array of something, for example UI buttons:

var buttons = [
  new OpenButton(),
  new ExportButton(),
  new SaveButton(),
  new DeleteButton()
];

Nice and clean code. But then the requirements change...

Marketing department decides, that ExportButton should only be shown to paying customers, and otherwise removed from the UI completely.

This might be very well be a good business decision, but from the code perspective it's not good at all. When the order of elements in array doesn't matter we could get away with a code like that:

var buttons = [
  new OpenButton(),
  new SaveButton(),
  new DeleteButton()
];
if (isPayingCustomer()) {
  buttons.push(new ExportButton());
}

But in our case the order definitely matters - UI buttons can't just be in any random order. And therefore we have to split the creation of array into multiple parts, which results in quite ugly code:

var buttons = [new OpenButton()];
if (isPayingCustomer()) {
  buttons.push(new ExportButton());
}
buttons.push(new SaveButton());
buttons.push(new DeleteButton());

We went from nice and clear declarative code into much harder to understand imperative code. That's a great price to pay for such a small change in functionality. Could we do it better?

Instead of conditionally pushing things to array, we would like to just declare that this array element only exists under certain conditions. We could express it like that:

var buttons = [
  new OpenButton(),
  isPayingCustomer() ? new ExportButton() : null,
  new SaveButton(),
  new DeleteButton()
];

That's all nice and clean, except that we didn't want any null elements in our array. The code that will use this array probably expects all array elements to be buttons, and we don't want to change it to ignore nulls.

Luckily there is compact() - a method of arrays that removes all null and undefined elements from array. That's exactly what we need! We just add a call to it at the end of our code:

var buttons = [
  new OpenButton(),
  isPayingCustomer() ? new ExportButton() : null,
  new SaveButton(),
  new DeleteButton()
].compact();

The only catch is that JavaScript arrays don't actually have a compact() method. But that's not a big problem - anyone can add it to Array.prototype and it's already there in some JavaScript libraries (e.g. Prototype). Here's a quick implementation using filter():

Array.prototype.compact = function() {
  return this.filter(function(x) {
    return x !== null && x !== undefined;
  });
};

compact() does something so simple that it rarely gets attention. But that simple tool has often saved me from writing some truly ugly push-push-push-code. I hope it helps you too.

Now go and compact those arrays!

Kirjutatud 13. jaanuaril 2010.

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Eesti Trinoloogide Maja. Eesti trinoloogiahuviliste avalik kogunemiskoht. info@triin.net

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