How immutability tamed the Wild West

Pure functions are good

The key to immutability is understanding the notion of a pure function. A pure function is one that always returns the same output for a given input. Pure functions are said to be deterministic, in that the output is 100% predictable based on the input. In simpler terms, a pure function is a function with no side effects. It will never change something behind your back.

We’ve all had this experience:

function addPie(items: string[]) {
  items.push("Apple Pie"); // side effect!
  return items;
}

const order = ["Burger", "Fries"];
const before = order;

const updated = addPie(order);

console.log("before:", before); // ["Burger", "Fries", "Apple Pie"] ← oops
console.log("updated:", updated); // ["Burger", "Fries", "Apple Pie"]

Note the addPie function, which is impure and thus has a side effect. It changes the items array you send it. As a result, the before reference changes as well. Not good—you might not expect that. When data is shared, being mutable turns everything into a moving target that is hard to hit.

But if the function provides immutability:

function addPieImmutable(items: string[]) {
  return [...items, "Apple Pie"]; // no side effects, new array
}

const order = ["Burger", "Fries"];
const before = order;

const updated = addPieImmutable(order);

console.log("before:", before);   // ["Burger", "Fries"] stable
console.log("updated:", updated); // ["Burger", "Fries", "Apple Pie"]

Here, the before reference remains unchanged. Because instead of updating the order, we created a new one (updated).

Change happens

Now this is a trivial example, but you can see how in the second version, there can never be a race condition or a battle for data because the order itself never changes. Instead, the order is recreated. Immutability doesn’t mean nothing changes; it means values never change once created. You still “change” by rebinding a name to a new value.

The notion of a “before” and “after” state is critical if you want features like undo, audit tracing, and other things that require a complete history of state.