Implement a stack data structure in JavaScript that contains the following operations:

• new Stack(): Creates an instance of a Stack class that doesn't contain any items. The constructor does not accept any arguments.
• push(): Pushes an item onto the top of the stack and returns the new length of the stack. Required time complexity: O(1).
• pop(): Removes and returns the item at the top of the stack, or returns undefined if the stack is empty. Required time complexity: O(1).
• isEmpty(): Determines if the stack is empty. Required time complexity: O(1).
• peek(): Returns the item at the top of the stack without removing it, or returns undefined if the stack is empty. Required time complexity: O(1).
• length(): Returns the number of items in the stack. Required time complexity: O(1).

Examples

const stack = new Stack();
stack.isEmpty(); // true
stack.push(1);
stack.push(2);
stack.length(); // 2
stack.push(3);
stack.peek(); // 3
stack.pop(); // 3
stack.isEmpty(); // false