What is the difference between `==` and `===` operators?

0-2 years experience

1. 1You need to check if a variable `userId` equals the string `'123'`. Which operator would you use and why?
2. 2If you write `if (value == null) { ... }`, what values will cause the block to run? Explain the behavior.
3. 3Given `let a = '0'; let b = 0;`, what will `a == b` and `a === b` evaluate to? Explain.

2-5 years experience

1. 1While debugging a feature, you notice that a condition using `==` is unexpectedly true for an object. Walk me through how you would investigate and fix it.
2. 2Your team wants to enforce consistent equality checks across the codebase. What trade‑offs would you consider when introducing a lint rule to ban `==`?
3. 3A JSON API returns numbers as strings, and a comparison using `===` fails. How would you handle this without introducing bugs elsewhere?

5-8 years experience

1. 1You are refactoring a large legacy module that heavily uses `==`. Describe a systematic approach to migrate to `===` while minimizing regression risk.
2. 2In a performance‑critical loop, you need to compare primitive values. Discuss any measurable impact of using `===` vs `==` and how you would benchmark it.
3. 3Your microservice communicates with a loosely typed client that sometimes sends booleans as `'true'` strings. How would you design a validation layer that correctly handles equality without relying on coercion?

8+ years experience

1. 1Your organization is planning a migration from a mixed JavaScript/TypeScript codebase to strict TypeScript. How would you address the widespread use of `==` in terms of tooling, code reviews, and long‑term maintainability?
2. 2When defining a shared utility library used across multiple teams, what guidelines would you set regarding equality operators, and how would you enforce them across CI pipelines?
3. 3Consider a large monorepo where some packages target older browsers that lack `Object.is`. How would you decide between using `===`, a polyfilled `Object.is`, or a custom equality helper for consistency?