Primitive vs reference types

Model answer. There are seven primitives: string, number, bigint, boolean, undefined, null and symbol. Everything else (plain objects, arrays, functions, dates, maps) is an object.

• Primitives are immutable and copied by value. let b = a copies the value, so changing b never affects a.
• Objects are copied by reference. The variable holds a reference, so two variables can point at the same object:

const a = { score: 1 };
const b = a;
b.score = 2;
console.log(a.score); // 2: same object

• Comparison: primitives compare by value ('hi' === 'hi' is true); objects compare by identity ({} === {} is false, because they are two different objects).

This matters when passing values to functions: a function can mutate an object you pass in, but it can never change a primitive argument as the caller sees it.