References: two names, one object

Assignment copies the reference, not the object

This lesson exists because the most confusing bug class in JavaScript is two variables secretly sharing one object. The actual rule: a variable never contains an object directly — it contains a reference, the object's address in memory. So const b = a copies the address, not the data. Afterward a and b point at the same object; change it through either name and the other sees it, because there is only one object.

Numbers, strings, and booleans do NOT behave this way: assigning them copies the value itself. Only objects and arrays (arrays are objects) are shared by reference.

The same rule explains === on objects: it compares references — "same object in memory?" — not contents. { a: 1 } === { a: 1 } is false: two separate objects that merely look alike.

To copy for real, build a new object or array: { ...obj } and [...xs] spread the contents into a fresh one. One warning: spread makes a shallow copy, one level deep, so objects nested inside are still shared. For a fully independent copy of nested data, use structuredClone(obj).

Python lists and dicts behave identically, so you may have met this bug before under the name aliasing.