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 based mostly on the enter. In less complicated phrases, a pure perform is a perform with no unwanted side effects. It’ll by no means change one thing behind your again.

We’ve all had this expertise:

perform 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"]

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

But when the perform supplies immutability:

perform 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 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 information as a result of the order itself by no means adjustments. As a substitute, the order is recreated. Immutability doesn’t imply nothing adjustments; 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 in order for you options like undo, audit tracing, and different issues that require an entire historical past of state.