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Enmanuel Durán
Enmanuel Durán

Posted on • Originally published at

Short-circuiting for faster evaluations in javascript

Before talking about Short-circuiting let’s have a quick reminder about boolean expressions, here a quick table I made to help you “Refresh” your memory:

True False False True
False True False True
True True True True
False False False False

Memory refreshed?, I know I know, most of you probably knew this already, but hey, I had to take precautions, excellent! Now we’re finally ready to start talking about the real thing.

Let’s take the following statement:

const animal = false;
const sound = 'woof';

if (sound === 'woof' && animal) {
    const dog = true;

If we analyze the expression above we’ll see that it’s making use of the && (AND) operator, which means that all statements englobing the condition must be true_in order for the expression to be _true. I want you to pay attention to the way you evaluate the condition above, we first evaluate sound === 'woof' and then animal which means that we evaluate conditions L -> R (left to right ), this is very basic but very important to understand.

Short-circuit means that statements are evaluated L -> R and tested in this order to avoid evaluating complete statement when possible, so basically:

false && anything is short-circuit evaluated to false

true || anything is short-circuit evaluated to true

Not clear yet?, let me enlight you based on the example above but switching the conditional arguments:

const animal = false;
const sound = 'woof';

// The if statement is short-circuit evaluated to false
if (animal && sound === 'woof') {
    const dog = true;

The condition above will end up being false because animal is false, this means that we don’t evaluate sound === 'woof' because we already know the condition is false which means that this condition has been short-circuit evaluated to false. That’s it, that is the essence of short-circuiting.

Still don’t see what is the advantage of knowing this?, Based on the concepts we’ve learned, we can improve our conditions and always evaluate the most important parameters in our conditions first, always evaluate first the variables you know are most likely to define your statements so that it can have more chances to be “short-circuited”, keep this in mind from now on and you’ll be making some small but good improvements to your application.

Did you like the article? in our previous article Ternary operators and boolean assignations we covered a topic that could be very helpful if combined with what we’ve learned today, don’t forget to share it with your friends and support us by subscribing to our email list, see you in the next one.

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