JavaScript Interview Questions

1. What is the difference between == and ===?

== checks for value equality with type coercion.
=== checks for both value and type without coercion.

'5' == 5   // true
'5' === 5  // false

2. What is a closure in JavaScript?

A closure is a function that retains access to its outer function’s variables even after the outer function has finished executing.

Closure – tai funkcija, kuri "prisimena" savo aplinką net ir tada, kai yra vykdoma už tos aplinkos ribų.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}
const counter = outer();
counter(); // 1
counter(); // 2

3. What is the difference between var, let, and const?

- var var is function-scoped, not block-scoped, and can be redeclared and updated.
- let is block-scoped, can be updated, but not redeclared in the same scope.
- const is also block-scoped, and cannot be reassigned or redeclared, but if it's an object or array, the contents can be mutated.

function test() {
  if (true) {
    var x = 1;
    let y = 2;
    const z = 3;
  }
  console.log(x); // 1
  console.log(y); // ReferenceError
  console.log(z); // ReferenceError
}