React JS

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React · State · Effects · Components

React JS

Practice original scenario-based React JS questions. Take a timed mock, focus on a topic, and learn from detailed explanations. These are independent practice materials, not official exam questions or a guarantee of certification readiness.

16 questions · 24 minutes · 70% practice target

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Topics you will practice

  • State & Identity
  • Effects & Async Work
  • State Architecture
  • Quality & Performance

Try a sample

Think through each question, then reveal the explanation. Full attempts save your answers and scores to your account.

A click handler calls setCount(count + 1) three times while count is 0. In this handler all calls read the same render snapshot. What is the resulting count after React processes them?

  1. 1
  2. 3
  3. 0
  4. An automatic TypeError
Show answer and explanation

Correct answer: 1

Each call queues replacement with 1 from the same snapshot. Use functional updates when the next value depends on earlier queued state updates.

A handler must increment state three times based on the preceding queued value. Which approach fits?

  1. Mutate count directly three times
  2. Call setCount(count + 1) and immediately read the variable as updated
  3. Set a DOM attribute and assume state changes
  4. Call setCount(c => c + 1) three times
Show answer and explanation

Correct answer: Call setCount(c => c + 1) three times

Updater functions receive the pending state in sequence. Direct mutation and DOM changes do not schedule React state correctly; the current render variable remains a snapshot.

A component mutates an object in state and passes the same reference back to its setter. The display may not update. What should change?

  1. Change only the object variable name
  2. Move every object into a global singleton
  3. Create a new object with the changed property and preserve other values
  4. Force a full browser reload after every mutation
Show answer and explanation

Correct answer: Create a new object with the changed property and preserve other values

React can skip updates when the state identity is unchanged. Creating a new value expresses the update; renaming references or using global mutation does not solve the state model.

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