Implement Queue using Stacks.js

/**
Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.
Notes:
You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).
*/

/**
 * @constructor
 */
var Queue = function() {
    this.stack1 = []; // come in
    this.stack2 = []; // for go out
};

/**
 * @param {number} x
 * @returns {void}
 */
Queue.prototype.push = function(x) {
    var len = this.stack2.length,
        i;
    
    for (i = 0; i < len; i++) {
        this.stack1.push(this.stack2.pop());
    }
    
    this.stack1.push(x);
    
    for (i = 0; i < len + 1; i++) {
        this.stack2.push(this.stack1.pop());
    }
};

/**
 * @returns {void}
 */
Queue.prototype.pop = function() {
    this.stack2.pop();
};

/**
 * @returns {number}
 */
Queue.prototype.peek = function() {
    var x = this.stack2.pop();
    
    this.stack2.push(x);
    return x;
};

/**
 * @returns {boolean}
 */
Queue.prototype.empty = function() {
    var len = this.stack2.length;
        
    if (len === 0) {
        return true;
    }
    
    return false;
};