Objects vs Primitives: JS Interview Cheatsheet  with Traps, and Boxing Explained

“Why can you call .toUpperCase() on a string, but can't add a custom property to it?” This question is used in interviews to check your understanding of primitives and objects in JavaScript. In this article, we break down 3 differences that will save you from hidden bugs: Storage , copying, and comparison — why two identical objects are never equal, and how passing by reference can betray you Mutability — how to accidentally overload your process with primitive copies (and when it actually matters) Methods and autoboxing — why primitives pretend to have methods , and why null and undefined refuse to play along Everything you need for confident answers in interviews and clean code on the job. Want to test your knowledge right away? 👉 Free mock interview based on this article - AI will ask you questions, check your answers, and point out gaps in your understanding of this topic. 5–10 minutes - and you'll know what you've mastered and what still needs work. Fundamental Concepts Mutability