How do I increment and decrement values in an object using JavaScript Classes?

Question

I am trying to do a simple Backpack class to review objects with vanilla javascript. While doing so, I've run into a block because I am unable to create a proper AddItem method or get my decrementItem method to work either.

My code is below:

class Backpack {
  constructor(name) {
    this.name = name;
    this.items = {};
  }

  name() {
    return `My backpack's name is ${this.name}`;
  }

  addItem(i, value) {
    return this.items[i] ? (this.items[i] += value) : (this.items[i] = 1);
  }

  decrementItem(i) {
    if (this.items[i]) {
    }
    return this.items[i] ? this.items[i]-- : (this.items[i] = 0);
  }
}

let newBackpack = new Backpack("Bacon");

newBackpack.addItem("pencil", 3);
newBackpack.addItem("book");
newBackpack.addItem("pen");
newBackpack.addItem("paper");

console.log(newBackpack.items);

I am also curious as to how I could add validation to my decrementItem method to check to see if a value doesn't go negative as that would cause some issues.

Answer 1

You could add these changes:

• i is often used in loops to refer to numeric indexes. It's not immediately clear that you use it for item names. Naming this variable item might make more sense.
• In addItem, when there is no such item to start with, you always add 1, discarding the passed value. Make sure to add the value
• In addItem, value seems to be an optional argument. You could add a default value of 1
• When altering a quantity, you can use this shorthand to make sure the previous value is used, or zero when there is none or it's already zero: this.items[item] || 0
• Math.max(n, 0) would allow you to make sure no value goes below zero

class Backpack {
  constructor(name) {
    this.name = name;
    this.items = {};
  }

  name() {
    return `My backpack's name is ${this.name}`;
  }

  addItem(item, value = 1) { // 1 by default if not provided
    return this.items[item] = (this.items[item] || 0) + value; // Add the value
  }

  decrementItem(item) {
    return this.items[item] = Math.max((this.items[item] || 0) - 1, 0); // 0 minimum
  }
}

let newBackpack = new Backpack("Bacon");

newBackpack.addItem("pencil", 3);
newBackpack.decrementItem("pencil");
newBackpack.addItem("book");
newBackpack.addItem("pen");
newBackpack.addItem("paper");

console.log(newBackpack.items);

Answer 2

you must first save item to constructor

class Backpack {
  constructor(name) {
    this.name = name;
    this.items = {};
  }

  name() {
    return `My backpack's name is ${this.name}`;
  }

  addItem(i, value) {
    this.items[i] = value ? value : 1;
  }

  decrementItem(i) {
    if (this.items[i]) {
      this.items[i] = this.items[i] - 1;
    } else {
      console.log(`Item not found !!`);
    }
  }
}

let newBackpack = new Backpack("Bacon");

newBackpack.addItem("pencil", 3);
newBackpack.addItem("book");
newBackpack.addItem("pen");
newBackpack.addItem("paper");

newBackpack.decrementItem("pencil");

console.log(newBackpack.items);

Answer 3

This might look nice for ur interview I guess

class Backpack {
  constructor(name){
    this.name=name
    this.items={}
    this.removeItem=function(item){
      item=item.toLowerCase()
      if(this.items[item]){delete(this.items[item])}
      else{throw Error("Cannot delete a NON-EXISTING attribute "+item)}
    }
    this.minusItem=function(item,number){
      item=item.toLowerCase()
      if(this.items[item]){this.items[item]-=number||1}
      else{throw Error("Cannot Subtract from a NON-EXISTING attribute "+item)}
    }
    this.addItem=function(item,number){
      item=item.toLowerCase()
      if(this.items[item]){this.items[item]+=number||1}
      else{this.items[item]=number||1}
    }
    this.getCountOfItems=function(item){
      item=item.toLowerCase()
      return this.items[item]||null
    }
  }
}

let newBackpack = new Backpack("Bacon");

newBackpack.addItem("pencil",3);
newBackpack.addItem("pencil",5);
newBackpack.minusItem("pencil",2);
newBackpack.addItem("book");
newBackpack.addItem("pen");
newBackpack.addItem("paper");

console.log(newBackpack.items);
console.log(newBackpack.getCountOfItems("pencil")) //6
console.log(newBackpack.getCountOfItems("lunchbox")) //null representing it not existing

try{newBackpack.minusItem("lunchbox",2)}
catch(err){console.error(err.message)}

try{console.log(newBackpack.removeItem("lunchbox"))}
catch(err){console.error(err.message)}