Spread vs rest syntax

Model answer. The same three dots do opposite jobs depending on where they appear:

• Spread expands an iterable or object into individual items, on the right-hand side or in a call.
• Rest collects the remaining items into one array or object, on the left-hand side or in a parameter list.

// spread
const merged = [...[1, 2], ...[3]];          // [1, 2, 3]
const settings = { ...defaults, theme: 'dark' };  // shallow copy + override
Math.max(...[4, 9, 2]);                       // 9

// rest
function sum(...nums) { return nums.reduce((a, b) => a + b, 0); }
const [first, ...others] = [10, 20, 30];      // first = 10, others = [20, 30]
const { password, ...safeUser } = user;       // drop one key

Rest must come last (function f(...a, b) is a syntax error). Object spread makes a shallow copy: nested objects are still shared.