A React re-render means React calls components to calculate what the UI should look like next. It does not mean the browser rebuilt the whole page or that every DOM node changed. React then commits only the DOM changes needed, and the browser paints the result.
What “re-render” means in React
React’s documentation puts it plainly: “Rendering is React calling your components.” (React, “Render and Commit”.) During rendering, components calculate a UI snapshot from their current props, state, and context. React uses that result to determine what the next component tree should show.
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Rendering is a calculation, not a synonym for changing the DOM. A component can run and produce the same output as before; in that case, React may have no DOM changes to commit. React says it “only changes the DOM nodes if there’s a difference between renders.” (React, “Render and Commit”.)
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The four stages from an update to what you see
| Stage | What happens | What it does not mean |
|---|---|---|
| Trigger | An initial mount or an update requests React work. | A DOM change has not necessarily happened. |
| Render | React calls relevant components to calculate the next UI output. | The whole DOM is not necessarily rebuilt or replaced. |
| Commit | React applies necessary changes to the DOM. | Every node is not necessarily changed. |
| Browser paint | The browser displays the resulting screen. | This is distinct from React calling a component. |
React describes screen updates as trigger, render, and commit; browser painting follows the DOM update. Rendering can traverse the component tree, but the distinction that matters is between calculating output and applying changes. A render alone does not establish that the DOM changed.
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What happens when a click updates state
- The handler runs with its render’s state snapshot. An event handler is created during a particular render, so it sees the state values from that render.
- The setter queues an update. Calling a state setter requests a later render; it does not rewrite the value already captured by the currently running handler. React explains this as state as a snapshot.
- React processes queued updates. Updates made during the same event handler are often batched, so React can process them together after the handler finishes. Separate intentional events, such as two distinct clicks, are processed separately. See Queueing a Series of State Updates.
- Components calculate the next output. React calls components as it works through the resulting tree to determine what should appear.
- React commits necessary DOM changes. It updates nodes where the calculated output differs; unchanged nodes need not be touched.
- The browser paints. The browser displays the screen after React updates the DOM.
Why state can still be the old value after a setter
The setter queues work for a subsequent render; it does not mutate the state snapshot used by the current function call. For example, if a click handler calls setCount(count + 1), then reads count on the next line, that read still refers to the value from the render that created the handler. The next render receives the updated value.
This also explains why two calls such as setCount(count + 1) in the same handler do not necessarily add two: both expressions use the same captured count value. When an update should build on the previous queued value, use the updater form, such as setCount(previous => previous + 1). React’s queueing guide explains how queued replacements and updater functions are processed.
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Why multiple component calls do not prove multiple DOM updates
Component execution belongs to the render phase; DOM mutations belong to the commit phase. In development, Strict Mode may call components more than once to help expose impure rendering logic. A pair of console messages is therefore not proof that React committed twice or that the browser redrew the whole page. Production behavior and development diagnostics are not interchangeable evidence.
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Render code should be pure: given the same inputs, it should calculate the same output without changing external state or causing side effects. Put side effects in the appropriate event handlers or Effects, rather than relying on a component function running a particular number of times.
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When React preserves state—and when it resets it
State is associated with a component’s identity in the render tree, not stored as an ordinary local variable that survives just because a function has the same name. React uses a component’s position, type, and key to determine whether it represents the same component as before. If identity is preserved, state can be preserved; if the tree position or identity changes, React may reset that state. The official guide covers these rules in Preserving and Resetting State.
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