How immutability tamed the Wild West

Pure capabilities are good

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

We’ve all had this expertise:

operate addPie(objects: string[]) {
  objects.push("Apple Pie"); // aspect impact!
  return objects;
}

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 operate, which is impure and thus has a aspect impact. It modifications the objects array you ship it. In consequence, the earlier than reference modifications as nicely. Not good—you won’t anticipate that. When information is shared, being mutable turns all the pieces right into a transferring goal that’s laborious to hit.

But when the operate offers immutability:

operate addPieImmutable(objects: string[]) {
  return [...items, "Apple Pie"]; // no unwanted 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"] steady
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 an alternative of updating the order, we created a brand new one (up to date).

Change occurs

Now it is a trivial instance, however you possibly can see how within the second model, there can by no means be a race situation or a battle for information 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 essential if you need options like undo, audit tracing, and different issues that require a whole historical past of state.