How to create duplicate objects in an array

This loop should do the trick:

for (var i = 3; i < 50; i++) {
  listings[i] = listings[i%3];
}

The magic is in the modular operator (%). i can increment to whatever you need it to but i%3 will keep returning 0, 1, and 2 in that order.

You could make a deep copy (clone) of each item using a method like this. We can avoid using % for this solution as it will serve no purpose ultimately.

jsFiddle

var i = 0;
while (listings.length < 50) {
    listings[listings.length] = clone(listings[i++]);
}
console.log(listings);


//https://stackoverflow.com/a/122190/1156119
function clone(obj){
    if(obj == null || typeof(obj) != 'object')
        return obj;

    var temp = obj.constructor(); // changed

    for(var key in obj)
        temp[key] = clone(obj[key]);
    return temp;
}

while(listings.length <= 50){
    listings[listings.length] = listings[listings.length - 3]
}

I prefer the while loop to the for loop for something like this.

var listings = [
    { 
        address: "123 41st St",
        bedrooms: 2,
        bathrooms: 2,
        image: "img/1bdrm_a.jpg"
    },
    { 
        address: "234 52nd St",
        bedrooms: 1,
        bathrooms: 1,
        image: "img/1bdrm_b.jpg"
    },
    { 
        address: "345 63rd St",
        bedrooms: 3,
        bathrooms: 2,
        image: "img/1bdrm_c.jpg"
    }
];
var len=listings.length;
for(var i=0;i<listings.length;i++){
    for(var j=i;j<50;j+=len){
        listings[j]=listings[i];
    }
}