Important topic of js for react:

If you want to actually survive React, your JavaScript game needs to be stronger than “I know console.log.”
Here are the JavaScript topics that matter most for React developers — no fluff, just the ones that will save you when React starts throwing curveballs:

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1. ES6+ Syntax (React breathes this stuff)

• let / const (block scope, immutability where needed)

• Arrow functions (this behavior, shorter syntax)

• Template literals (`${value}` for dynamic strings)

• Destructuring (arrays & objects, especially in props/state)

• Spread & rest operators (...obj, ...arr)

• Default parameters in functions

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2. Functions & Closures (Hooks rely on these)

• Function declarations vs expressions

• Higher-order functions (functions that take/return functions)

• Closures (why your useState value doesn’t update immediately)

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3. Array Methods (Rendering lists in React = 80% arrays)

• .map() (render lists)

• .filter() (remove items)

• .reduce() (accumulate values, e.g., totals)

• .forEach() (iteration without return)

• .find() & .findIndex() (locating items)

• .some() & .every() (boolean checks)

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4. Objects & Manipulation (State updates without mutation)

• Object destructuring

• Shallow vs deep copy

• Merging objects ({ ...obj1, ...obj2 })

• Dynamic keys ([keyName]: value)

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5. Asynchronous JavaScript (Data fetching in React)

• Promises (then, catch, finally)

• async / await syntax

• Error handling with try/catch

• fetch() basics

• Promise.all / Promise.race

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6. DOM & Events (React abstracts it, but you must know the basics)

• Query selectors (getElementById, querySelector)

• Event bubbling & capturing

• Preventing default behavior (e.preventDefault())

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7.  this Keyword (React class components & event handlers)

• How this works in different contexts

• Binding vs arrow functions in class components

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8. Modules & Imports/Exports

• Named exports vs default exports

• Importing functions, objects, and components

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9. JSON & Data Structures

• JSON.parse() & JSON.stringify()

• Working with nested objects & arrays

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10. Optional but Gold for Pro React Devs

• Debouncing & throttling (for performance)

• LocalStorage / SessionStorage

• Set, Map, WeakMap, WeakSet

• Generators & Iterators (rare, but handy)

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If you nail these, React will feel way less mysterious.