JavaScript quiz

More questions about JavaScript

These visible examples show the question style. LearnClash turns the full topic into ranked duels and spaced repetition.

1. 1 Easy

   How is JavaScript technically related to the Java programming language?

   Answer: They are unrelated

   Why: The resemblance is skin deep: one was built from Scheme and Self ideas, and the Java-like syntax was imposed by management.

2. 2 Medium

   What is the result of checking typeof null in JavaScript?

   Answer: "object"

   Why: It is a bug from the very first implementation, kept ever since because fixing it would break too much existing code.

3. 3 Hard

   What behavior moves variable declarations to the top of their scope?

   Answer: hoisting

   Why: The declarations only appear to move, and block-scoped ones are lifted too, but they sit unusable in the temporal dead zone until initialised.

4. 4 Easy

   Which built-in function shows a popup with a message and only an OK button?

   Answer: alert()

   Why: Browsers offer three script dialogs: this one has just OK, while confirm adds Cancel and prompt adds a text field.

5. 5 Medium

   In JavaScript, what is the official data type of Not-a-Number (NaN)?

   Answer: number

   Why: NaN is the only value not equal to itself, so the reliable check is Number.isNaN; the older global isNaN converts its argument first.

6. 6 Hard

   Which organization maintains the official standard that JavaScript is based on?

   Answer: Ecma International

   Why: The specification is numbered ECMA-262, and the technical committee that writes it is known as TC39.