QReact · Interview preparation
Does a React render always update the DOM?
No.
The idea to remember.
No. Rendering calculates the next UI description, while the commit phase applies only the changes React determines are needed. Measuring DOM work and React render work separately can reveal different bottlenecks.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: understand a render
01 / BEGINNERA render calculates UI; it does not always change the DOM.
function Greeting({ name }) {
console.count('Greeting rendered');
return <h2>Hello, {name}</h2>;
}Intermediate: memoize derived work
02 / INTERMEDIATEMemoize an expensive derived value when profiling justifies it.
const visibleRows = React.useMemo(
() => rows.filter(row => row.label.includes(query)),
[rows, query]
);Real scenario: stabilize props
03 / REAL SCENARIOOnly optimize a frequently updated dashboard after measuring.
const Chart = React.memo(function Chart({ data }) {
return <Graph data={data} />;
});
function Dashboard({ records, filter }) {
const data = React.useMemo(() => aggregate(records, filter), [records, filter]);
return <Chart data={data} />;
}02Check your understanding
Try it in your own words.
Explain does a React render always update the DOM without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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