Double Ended Queue (Deque) in Javascript

Deque.js

function Deque() {
    var rev = []
    var fwd = []

    this.size = function() {
        return rev.length + fwd.length
    }

    this.push_front = function(itm) {
        rev.push(itm)
    }

    this.push_back = function(itm) {
        fwd.push(itm)
    }

    this.pop_front = function() {
        //assert(this.size() > 0)
        if (rev.length == 0) {
            while (fwd.length > 0) {
                var itm = fwd.pop()
                rev.push(itm)
            }
        }
        return rev.pop()
    }

    this.pop_back = function() {
        //assert(this.size() > 0)
        if (fwd.length == 0) {
            while (rev.length > 0) {
                var itm = rev.pop()
                fwd.push(itm)
            }
        }
        return fwd.pop()
    }

    this.toList = function() {
        var lst = []
        for (var i = 0; i < rev.length; i++) {
            var j = rev.length - 1 - i
            lst.push(rev[j])
        }
        for (var i = 0; i < fwd.length; i++) {
            lst.push(fwd[i])
        }
        return lst
    }
}