React State Updates: Compare Replacement Values with Pure Updater Functions

Fix the starting point and the event boundary

Use a local counter component with count starting at 0. Prepare two separate cases and reset the component to the same initial state before each. Compare updates made inside one click handler, rather than comparing several separate clicks. Keep the page free of external requests and form submissions.

This supports React state and hooks foundations for learners in Jaipur. It is a proposed counter lab, not a production performance benchmark or a claim about a particular course timetable.

Predict repeated replacement values

In the first handler, call setCount(count + 1) three times. Each expression reads count from that render's snapshot, so each computes the same replacement value, 1. With this isolated case starting at 0, the next displayed count is 1 rather than 3. The setter does not synchronously rewrite the local count binding.

Compare a queued transformation

In the second handler, call setCount(current => current + 1) three times. React can apply each queued updater to the pending result of the previous update. Starting from 0, the intended sequence is 1, 2, 3 and the next displayed count is 3.

The function expresses a transition from the pending previous value. It is useful when the new state is calculated from that value; it is not a reason to make every unrelated state update more complicated.

Keep each updater pure

Return the next value without mutating existing state, sending a request, writing a log as an application action or modifying another variable. Development Strict Mode can call updater functions twice to help find accidental impurities. An updater therefore should not be used as a place to perform a one-time side effect.

Save observations that answer the question

• The initial count for each independent case.
• Whether the calls occurred in one handler or separate events.
• The expected next displayed value and the actual displayed value.
• A one-sentence explanation of replacement versus transformation.

Apply the idea with restraint

A real interface may also need validation, reducer-style transitions or a different state structure. Begin with the dependency: does this next value depend on the pending previous value? The related Mango Engineers React learning path covers broader frontend foundations; confirm current prerequisites and coverage before choosing a course.