SOURCE

function Stack(){
    this.dataStore = [];
    this.top = 0;
    this.push = push;
    this.pop = pop;
    this.peek = peek;
    this.clear = clear;
    this.length = length;
}

function push(element){
    this.dataStore[this.top++] = element;
}

function pop(){
    return this.dataStore[--this.top];
    
}

function peek(){
    return this.dataStore[this.top-1];
}

function clear(){
    this.top = 0;
}

function length(){
    return this.top;
}


var a = new Stack();
a.push('alice');
a.push('ben');
a.push('jack');
a.push('lily');
console.log('length:' + a.length());
console.log(a.peek());
console.log(a.pop());
console.log(a.peek());
console.log('length:' + a.length());