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 primarily based on the enter. In less complicated 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"]
Word the addPie operate, which is impure and thus has a facet impact. It modifications the gadgets array you ship it. Because of this, the earlier than reference modifications as properly. Not good—you may not anticipate that. When information is shared, being mutable turns all the things right into a shifting goal that’s laborious to hit.
But when the operate offers 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"] 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 may 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 crucial if you’d like options like undo, audit tracing, and different issues that require an entire historical past of state.
