push() vs concat()

Model answer.

• `push(...items)` adds items to the end of the same array (mutates it) and returns the new length.
• `concat(...values)` returns a new array made of the original plus the values; the original is unchanged. Arrays passed to concat are flattened one level.

const a = [1, 2];
a.push(3);           // returns 3; a is [1, 2, 3]
const b = a.concat([4, 5], 6);   // b is [1, 2, 3, 4, 5, 6]; a unchanged
a.push([4, 5]);      // a is [1, 2, 3, [4, 5]]: push does not flatten

A common bug is writing list = list.push(x), which replaces the array with a number. When you need immutability (for example React state), use concat or spread: setItems([...items, x]).