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React is an extremely popular framework for constructing user interfaces (UIs). A React UI is organized as a tree of components, each of which is defined by a function that returns a literal written in JSX, a syntactic extension of JavaScript consisting of a combination of XML tags, executable JavaScript code, and references to sub-components. React supports incremental re-rendering by maintaining an in-memory representation of a web page’s Document Object Model (DOM) and automatically calculating a set of minimal changes that must be applied to the DOM when state changes occur. However, React’s semantics are complex and subtle, and programmers often write code that gives rise to unnecessary re-rendering, which hurts performance and responsiveness. We identify 5 React anti-patterns that give rise to unnecessary re-rendering, present a static analysis for detecting them, and rewrite rules that suggest how to refactor the code to improve rendering performance. The static analysis is potentially unsound, so developers should carefully review the suggested refactorings. A survey of 7,758 React repositories showed that 92.1% of them exhibit at least one anti-pattern, and careful experimental evaluation on 23 React projects revealed that the suggested refactoring reduces the number of rendering operations by 33.3% on average while preserving application behavior in all but one case. With a small increase in code complexity, we find an average reduction in rendering time of 20.54%, and three case studies reveal that the refactorings can greatly improve application responsiveness as the number of components scales.