JavaScript and React quiz

More questions about JavaScript and React

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

1. 1 Easy

   What animal is on the cover of the JavaScript 'Definitive Guide'?

   Answer: Rhino

   Why: The Mozilla Rhino JavaScript engine is named after the rhinoceros on this O'Reilly book's cover.

2. 2 Medium

   Why does React require keys when rendering list items?

   Answer: Efficiently update elements

   Why: React keeps key to itself: your component never receives it as a prop, so pass the ID separately if needed.

3. 3 Hard

   Which hook provides a way to access DOM nodes directly?

   Answer: useRef

   Why: A ref is a plain object with one current property, and changing it never triggers a re-render, unlike state.

4. 4 Easy

   Which JavaScript keyword declares a variable that cannot be reassigned?

   Answer: const

   Why: const locks the variable, not its contents, so you can still push items into a const array.

5. 5 Medium

   What three-dot notation allows expanding an array or object?

   Answer: Spread operator

   Why: Spreading arrays arrived with ES2015, but spreading objects had to wait until ES2018.

6. 6 Hard

   What allows a function to remember its lexical scope?

   Answer: Closure

   Why: A closure keeps its outer variables reachable even after the outer function returns, which is how JavaScript can emulate private variables.