Learning about data structures in details. Checked few js tutorials online and they seemed to use array for everything.
Like this:
class Stack { 
    // Array is used to implement stack 
    constructor() 
    { 
        this.items = []; 
    } 
    // Functions to be implemented 
    // push(item) 
    // pop() 
    // peek() 
    // isEmpty() 
    // printStack() 
} 
 
    