How immutability tamed the Wild West

Pure capabilities are good

The important thing to immutability is knowing the notion of a pure perform. A pure perform is one which all the time returns the identical output for a given enter. Pure capabilities are stated to be deterministic, in that the output is 100% predictable primarily based on the enter. In less complicated phrases, a pure perform is a perform with no uncomfortable side effects. It can by no means change one thing behind your again.

We’ve all had this expertise:

perform addPie(gadgets: string[]) {
  gadgets.push("Apple Pie"); // facet impact!
  return gadgets;
}

const order = ["Burger", "Fries"];
const earlier than = order;

const up to date = addPie(order);

console.log("earlier than:", earlier than); // ["Burger", "Fries", "Apple Pie"] ← oops
console.log("up to date:", up to date); // ["Burger", "Fries", "Apple Pie"]

Notice the addPie perform, which is impure and thus has a facet impact. It modifications the gadgets array you ship it. In consequence, the earlier than reference modifications as properly. Not good—you won’t count on that. When knowledge is shared, being mutable turns all the pieces right into a transferring goal that’s laborious to hit.

But when the perform supplies immutability:

perform addPieImmutable(gadgets: string[]) {
  return [...items, "Apple Pie"]; // no uncomfortable side effects, new array
}

const order = ["Burger", "Fries"];
const earlier than = order;

const up to date = addPieImmutable(order);

console.log("earlier than:", earlier than);   // ["Burger", "Fries"] secure
console.log("up to date:", up to date); // ["Burger", "Fries", "Apple Pie"]

Right here, the earlier than reference stays unchanged. As a result of as a substitute of updating the order, we created a brand new one (up to date).

Change occurs

Now this can be a trivial instance, however you’ll be able to see how within the second model, there can by no means be a race situation or a battle for knowledge as a result of the order itself by no means modifications. As a substitute, the order is recreated. Immutability doesn’t imply nothing modifications; it means values by no means change as soon as created. You continue to “change” by rebinding a reputation to a brand new worth.

The notion of a “earlier than” and “after” state is important if you’d like options like undo, audit tracing, and different issues that require a whole historical past of state.