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

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.

Solution

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);