Checking whether a bullet has hit a Space Invader
Welcome to CodeReview! 

I'm not really working with javascript, but I noticed some things I'd like to mention: 


You can safely move the first two initialisations of invaderX an invaderY one condition deeper into the Invaders[i].isVisible() block - if the invader isn't visible there is no need to know the invaders position. 
Same with the score variable. Initialise the variable immediately before using it (game.updateScore(score)).
Don't use magic numbers like 15 for the radius of an invader. Use a static class constant or instance property of an Invader instance. This would even allow you to create invaders of different size. 
I would also put the isInInvadersRadius condition into a separate function. This function could look something like this:

function areInRadius(location1, location2, radius) {
    var x1 = location1[0],
        y1 = location1[1],
        x2 = location2[0],
        y2 = location2[1];

    return x1 &gt;= x2 - radius &amp;&amp; x1 &lt;= x2 + radius &amp;&amp;
           y1 &gt;= y2 - radius &amp;&amp; y1 &lt;= y2 + radius;
}


Using this function you wouldn't even need the temporaries bulletX and bulletY, just calling areInRadius(bullet.location(), Invaders[i].location(), Invader[i].radius). 

Your function would look like this:

function testForInvaders(bullet) {
    for(var i = 0; i &lt; Invaders.length; i++) {
        if(Invaders[i].isVisible() &amp;&amp; areInRadius(bullet.location(), Invaders[i].location, Invaders[i].radius) {
            Invaders[i].removeVisibility();

            var score = document.getElementById('score');
            game.incrementScore();
            game.updateScore(score);
            return false;
        }
    }

    return true;
}

testForInvaders gets called quite often as you already mentioned and I'm honestly not sure if the overhead of the function call above will screw with the performance. You should consider to measure the impact.


Since I'm not into javascript I can't say anything special about hit detection (maybe there is a builtin or an standard way of doing so), but I'm sure other users will remind you of those (if existent). 

Nice piece of code you delivered here for your first review. Keep it up!