[Javascript] Avoiding Mutations in JavaScript with Immutable Data Structures

To demonstrate the difference between mutability and immutability, imagine taking a drink from a glass of water. If our glass is mutable, when we take a drink, we retain the same glass and change the amount of water in that glass. However, if our glass is immutable, when we take a drink, we get back a brand new, identical glass containing the correctly drank amount. Perhaps a strange way to conceive of the action, but creating new data structures makes our methods pure and thread-safe, a benefit of functional programming.

class MutableGlass {
  constructor(content, amount) {
    this.content = content
    this.amount = amount
  }

  takeDrink(value) {
    this.amount = Math.max(this.amount - value, 0)
    return this
  }
}

// We can verify this by checking the references of the first glass and
// the glass returned by `takeDrink()` and see that they are the same.
const mg1 = new MutableGlass('water', 100)
const mg2 = mg1.takeDrink(20)
console.log(mg1.amount === 80 && mg1.amount === mg2.amount) // true
console.log(mg1 === mg2) // true

Immutable class, whch every time should return a new instance:

// Taking a drink from the immutable glass returns an entirely new glass,
// but with the correct content and amount of it in the glass.
class ImmutableGlass {
  constructor(content, amount) {
    this.content = content
    this.amount = amount
  }

  takeDrink(value) {
    return new ImmutableGlass(this.content, Math.max(this.amount - value, 0))
  }
}

// We can verify this by checking the references and seeing that they are
// _not_ equal
const ig1 = new ImmutableGlass('water', 100)
const ig2 = ig1.takeDrink(20)
console.log(ig1.amount !== ig2.amount) // true
console.log(ig1 === ig2) // false