meet it

Pure functions return new data instead of changing old data.

A only reads its arguments and returns a result. It never mutates what it was given or anything outside it. Same input, same output, every time, which makes it a joy to test.

Anything else a function does is a . To update without mutating, build a copy with spread: { ...cart, owner: "Al" } copies the top level and changes one property. Spread is shallow, so for nested data use structuredClone, which makes a .

const cart = { owner: "Jo", items: ["pen"] };
function addItem(oldCart, item) {
  return { ...oldCart, items: [...oldCart.items, item] };
}
const newCart = addItem(cart, "ink");
console.log(cart.items, newCart.items);
const backup = structuredClone(newCart);
backup.items.push("pad");
console.log(newCart.items, backup.items);

Undo buttons and history features rely on this: every old version is still there, untouched.

guess first

A discount with a side effect. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

function withDiscount(order) {
  order.total = order.total * 0.5;
  return order;
}
const original = { id: 7, total: 40 };
const sale = withDiscount(original);
console.log(original.total, sale.total, sale === original);

your turn

Your turn. Level up without mutating.

The player is frozen, so mutating it would throw. Fill in a new level that is one more than the old one, using spread to copy the rest.

Your code

press run to see what happens

fix it

The copy changed the original. Can you fix it?

Broken code to fix

press run to see what happens

copy is a new object, so why did week.tasks change as well?
Spread is shallow. The copy holds the very same tasks array as the original.
Make a deep copy instead: const copy = structuredClone(plan);.

nailed it

you change data without breaking it. :D

Pure functions return new data and change nothing else.
Spread makes shallow copies; nested data is still shared.
structuredClone makes a deep copy of nested data.

new words in your collection

pure function
side effect
deep copy

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