References.

Eleventh class. Objects and arrays are passed by reference. Copying the name does not copy the data:

const a = [1]; const b = a; b.push(2); // a changes too

Shallow copies: [...arr], arr.slice(), Object.assign({}, obj). A push through an alias also changes the original; a push to the copy leaves the original length unchanged.

Print words like copy / same / changed to show the difference.

Common mistake: const copy = original for an object — that is the same reference.