Check for false

You can use something simpler:

if(!var){
    console.log('var is false'); 
}

If you want to check for false and alert if not, then no there isn't.

If you use if(val), then anything that evaluates to 'truthy', like a non-empty string, will also pass. So it depends on how stringent your criterion is. Using === and !== is generally considered good practice, to avoid accidentally matching truthy or falsy conditions via JavaScript's implicit boolean tests.

If you want an explicit check against false (and not undefined, null and others which I assume as you are using !== instead of !=) then yes, you have to use that.

Also, this is the same in a slightly smaller footprint:

if(borrar() !== !1)