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 easier phrases, a pure operate is a operate with no negative 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"]

Observe the addPie operate, which is impure and thus has a aspect impact. It adjustments the objects array you ship it. Because of this, the earlier than reference adjustments as nicely. Not good—you won’t count on that. When knowledge is shared, being mutable turns every part right into a shifting goal that’s laborious to hit.

But when the operate gives immutability:

operate addPieImmutable(objects: string[]) {
  return [...items, "Apple Pie"]; // no negative 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 knowledge as a result of the order itself by no means adjustments. As an alternative, 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 if you would like options like undo, audit tracing, and different issues that require an entire historical past of state.