Question 1

How do I explain JavaScript closures with examples?

Accepted Answer

A closure is a function that remembers its lexical scope even after the outer function returns. The classic example is a counter factory where inner methods increment a private `count` variable, demonstrating data privacy and the module pattern.

Question 2

What is the difference between == and === in JavaScript?

Accepted Answer

`==` performs type coercion before comparison, so `"5" == 5` is true, while `===` compares without coercion, making `"5" === 5` false. The recommended rule is to always use `===` unless you have a specific reason for loose equality.

Question 3

Why does Promise.then run before setTimeout with zero delay?

Accepted Answer

Promise callbacks are microtasks, which the event loop executes immediately after the current synchronous code finishes. setTimeout callbacks are macrotasks, which only run after all microtasks complete, so Promises always win the race.

Question 4

Does const make JavaScript objects immutable?

Accepted Answer

No, const only prevents reassignment of the variable binding, not mutation of the object's properties. You can still add or modify properties on a const object; use Object.freeze() to make it truly immutable.

Question 5

When should I use nullish coalescing instead of logical OR?

Accepted Answer

Use `??` when you want defaults only for null or undefined, since `||` also treats falsy values like 0 and empty strings as missing. For example, `0 ?? "default"` returns 0, while `0 || "default"` returns "default".

javascript-mastery

SYSTEM DOCUMENTATION & REQUIREMENTS

Frequently Asked Questions about javascript-mastery

Related Skills

javascript-mastery

test-shipped-code

go-linting

typescript-strict-patterns