Pure features 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 features 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 unintended effects. It is going to by no means change one thing behind your again.
We’ve all had this expertise:
operate 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 operate, 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 nicely. Not good—you may not anticipate that. When knowledge is shared, being mutable turns all the pieces right into a transferring goal that’s arduous to hit.
But when the operate gives immutability:
operate addPieImmutable(gadgets: string[]) {
return [...items, "Apple Pie"]; // no unintended 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 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 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 would like options like undo, audit tracing, and different issues that require an entire historical past of state.
