JavaScript Core Syntax 2

Question 1

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

Accepted Answer

`==` checks value only; `===` checks value and type

Answer

`==` performs type coercion before comparison (loose equality), while `===` checks both value and type without coercion (strict equality). For example, `'5' == 5` is `true` but `'5' === 5` is `false`.

Question 2

What does the `typeof` operator return for `null`?

Accepted Answer

'object'

Answer

`typeof null` returns `'object'`. This is a well-known quirk in JavaScript originating from its original implementation. `null` is a primitive value, but `typeof null === 'object'` is a long-standing bug in the language.

Question 3

What is the output of `typeof undefined`?

Accepted Answer

'undefined'

Answer

`typeof undefined` returns the string `'undefined'`. This is consistent and expected — `undefined` is the type of variables that have been declared but not assigned.

Question 4

What is the result of `0.1 + 0.2 === 0.3` in JavaScript?

Accepted Answer

false

Answer

Due to floating-point precision issues in IEEE 754 binary representation, `0.1 + 0.2` evaluates to `0.30000000000000004`, which is not strictly equal to `0.3`. So the expression is `false`.

Question 5

What does the `in` operator do in JavaScript?

Accepted Answer

Checks if a property exists in an object or its prototype chain

Answer

The `in` operator returns `true` if the specified property exists in the given object or anywhere in its prototype chain.

JavaScript Practice Test

JavaScript Practice Test

JavaScript Core Syntax 2