myArray.js

class OurArray {
    constructor () {
        this.length = 0;
        this.data = {};     // An array in JavaScript is Object
    }

    /** Lookup: Time Complexity = O(1)
     * @param {number} index
     * @return {any} item
    */
    get (index) {
        return this.data[index];
    }

    /**
     * Time Complexity = O(1) 
     * @param {any} item
     * @return {number} length 
     *    New length of the array
    */
    push (item) {
        this.data[this.length] = item;
        this.length++;
        return this.length;     // JavaScript `push` method returns the new length of the array
    }

    /** 
     * Time Complexity = O(1) 
     * @return {number} lastItem
    */
    pop () {
        const lastItem = this.data[this.length - 1];
        delete this.data[this.length - 1];
        this.length--;
        return lastItem;
    }

    /**
     * Time Complexity = O(N)
     * @param {number} index 
     *    Index of the item to be deleted
     * @return {any} item 
     *    Deleted item
    */
    deleteAtIndex (index) {
        const item = this.data[index];
        this.shiftItems(index);     // Shift the elements
        return item;
    }
  
    /**
     * Called from deleteAtIndex
     * Shift Items towards left from the index where the item has to be deleted.
     * @param {number} index
    */
    shiftItems(index) {
        for (let i = index; i < this.length - 1; i++) {
            this.data[i] = this.data[i + 1];    // The item at index is replaced.
        }
        delete this.data[this.length - 1];      // Removing the last item.
        this.length--;
    }
}

const myThings = new OurArray();

myThings;                               // { length: 0, data: {} }

myThings.get(0);                        // undefined

myThings.push('Books');                 // 1
myThings.push('Car');                   // 2

myThings.pop();                         // 'Car'

myThings.push('House');                 // 2
myThings.deleteAtIndex(1);              // 'House'

myThings.get(0);                        // 'Books'

myThings;                               // { length: 1, data: { '0': 'Books' } }