Javascript logical operator && in statements?

Question

I've starting to dig my teeth into backbone and when working with a router function creating and destroying views I came across this little snippet, which didn't quite make sense to me as I've always assumed these sorts of operators only work in conditional statements,

this.view && this.view.remove();

It was inside of a method attached to the router which seems to work like a charm, but I'm always weary of voodoo code that doesn't sit well with me.

Heres the full method

loadView : function(view) {
        this.view && this.view.remove();
        this.view = view;
    }

Would love to understand this a bit better, hope its not too silly to ask.

Cheers.

Answer 1

In JavaScript:

this.view && this.view.remove();

is equivalent to

this.view ? this.view.remove() : this.view;

The righthand side is only executed if the lefthand side is truthy . If the value of the expression is ignored and the lefthand side has no side-effects (as in this case), then it's also equivalent to this:

if (this.view) {
    this.view.remove();
}

So, in this specific case if there is an existing view , then it is removed. Note that the value of this expression is not necessarily a boolean (unlike in many other languages); it's whatever is returned by remove . However, this value is ignored, so it's not actually pertinent.

Answer 2

The operator && in javascript evaluates the first expression, if it is falsy it is returned, if it is truthy the second expression is evaluated and returned.

For || it is the other way round: the first expression is evaluated, if it is truthy it is returned, else the second expression is evaluated and returned.