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JavaScript

Chris Kwong
Chris Kwong
10,266 Points

What does "||" mean in javascript in a variable declaration? Example var variable = a || b;

I was looking at some code in a forum and this:

current = newGallery || 0;

popped up. I think it means: if newGallery is not null or undefined, set it to 0, otherwise current = newGallery.

What is this syntax? Is there an equivalent with &&. I only know of || and && as boolean OR and AND

2 Answers

Well, it's my understanding that || is going to be that Boolean "or," so this is going be "set current to either the variable called newGallery or zero," I believe!

It's a good idea to do a test to see how this works. It's not specifically looking for 'null' or 'undefined' it's actually looking for truthy/falsey. James Padolsey has an excellent write up on Truthy & Falsey worth reading.

In JS there are 5 falsey values: undefined, null, NaN, 0, "" (empty string), and false, of course.

So in a simple test we can look at a truthy equivalent:

    // Truthy
    var newGallery = 1;
    var current = newGallery || 'false';
    alert(current); // Alert returns 1

and falsey:

    // Falsey
    var newGallery = 0;
    var current = newGallery || 'false';
    alert(current); // Alert returns false
Chris Kwong
Chris Kwong
10,266 Points

Actually, you reversed it

the point is to set a default value when newGallery is null or undefined. I am just unsure of the syntax since I've only learned || as OR in a boolean statement, not in a variable declaration.

var defaultValue = 3;
var newValue = oldValue || defaultValue;

so if oldValue is null or undefined, it is supposed to be set to defaultValue of 3. But I am confused of the syntax. Is it a boolean statement now? What if oldValue is 0, is that falsey?

I'm not sure what you mean by reversed, did you run the tests in your browser? I just wanted to demonstrate the truthy/falsey. If newGallery equates to a falsey value then it will alert 'false', if it doesn't, it will use the defined value. The || is still an OR operator even when used in a variable declaration. It's basically saying "do this OR that" depending on what the variable value is determined to be.

Your example would immediately set newValue = 3 because you don't have an oldValue defined anywhere, so it would be falsey. As a side note you'd also need the variable oldValue defined or else it would throw an error.

// oldValue is falsey, so newValue = 3     
    var oldValue;
    var defaultValue = 3;
    var newValue = oldValue || defaultValue;
    alert(newValue);