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It tells React that this component's state changed, and React schedules a re-render where the component function runs again with the new value.",{"id":146,"topic":9,"difficulty":129,"body":147,"options":148,"correct_key":136,"explanation":157},"019fa9e5-7287-7e33-b1d1-48b2d2800197","A component has `const [user, setUser] = useState({ name: 'Ada', age: 30 });`. Calling `setUser({ age: 31 })` is run to update just the age. What happens to `name`?",[149,151,153,155],{"key":133,"text":150},"React automatically merges the new object with the old one, so `name` is preserved.",{"key":136,"text":152},"The state becomes `{ age: 31 }` — `name` is lost, because `useState` replaces.",{"key":139,"text":154},"React throws a runtime error because the object is incomplete.",{"key":142,"text":156},"`name` stays `'Ada'` but becomes read-only for the rest of the component's life.","Unlike class component `this.setState`, the `useState` setter replaces the whole value. To update one field you must spread the previous state: `setUser(prev => ({ ...prev, age: 31 }))`.",{"id":159,"topic":9,"difficulty":129,"body":160,"options":161,"correct_key":136,"explanation":170},"019fa9e5-728c-7720-9595-51363398e1e6","What does `useEffect(() => { fetchData(); }, [])` (empty dependency array) do?",[162,164,166,168],{"key":133,"text":163},"Runs `fetchData` before every render, including the first.",{"key":136,"text":165},"Runs `fetchData` once, right after the component mounts.",{"key":139,"text":167},"Never runs `fetchData` because the array is empty.",{"key":142,"text":169},"Runs `fetchData` once per second automatically.","An empty dependency array means the effect has no reactive dependencies, so React only runs it once after the initial mount, and never again on subsequent re-renders (unless the component unmounts and remounts).",{"id":172,"topic":9,"difficulty":173,"body":174,"options":175,"correct_key":139,"explanation":184},"019fa9e5-728d-7257-9a5a-c9686589e9ae",2,"`useEffect(() => { console.log('render'); });` is written with no second argument at all (no dependency array). When does it run?",[176,178,180,182],{"key":133,"text":177},"Only once, on mount, exactly like an empty array.",{"key":136,"text":179},"Only when the component is about to unmount.",{"key":139,"text":181},"After every single render, including the first and every re-render.",{"key":142,"text":183},"Never, because omitting the array is invalid and React ignores it silently.","Omitting the dependency array entirely (not even passing `[]`) tells React to run the effect after every render — mount and every subsequent re-render — with no memoization at all.",{"id":186,"topic":9,"difficulty":173,"body":187,"options":188,"correct_key":133,"explanation":197},"019fa9e5-728e-7113-919a-d608b3a102cd","```jsx\nuseEffect(() => {\n  const sub = eventEmitter.addListener('update', onUpdate);\n  return () => sub.remove();\n}, []);\n```\nWhen does the returned function `() => sub.remove()` run?",[189,191,193,195],{"key":133,"text":190},"Right before the component unmounts, or before the effect re-runs.",{"key":136,"text":192},"Immediately, synchronously, right after `addListener` is called.",{"key":139,"text":194},"Only if the app crashes and React needs to recover.",{"key":142,"text":196},"Every time any sibling component re-renders.","The function returned from an effect is its cleanup. React calls it right before running the effect again (not applicable here since deps are `[]`) and, in all cases, right before the component unmounts — the standard place to remove listeners registered by the effect.",{"id":199,"topic":9,"difficulty":173,"body":200,"options":201,"correct_key":142,"explanation":210},"019fa9e5-728e-7ae9-8971-90194d0cb5db","```jsx\nfunction Timer({ label }) {\n  useEffect(() => {\n    const id = setInterval(() => {\n      console.log(label);\n    }, 1000);\n    return () => clearInterval(id);\n  }, []);\n}\n```\n`label` changes over time via a prop update, but the console keeps logging the very first value it saw. Why?",[202,204,206,208],{"key":133,"text":203},"`console.log` caches its first argument forever across all calls.",{"key":136,"text":205},"`setInterval` in React Native ignores closures entirely.",{"key":139,"text":207},"This is a compiler error and the code would not actually run.",{"key":142,"text":209},"The effect's empty deps mean the closure over `label` is only created once.","Because the effect only runs once (`[]`), the callback passed to `setInterval` closes over the `label` value from that first render only. Later renders create a new `label` variable, but the already-running interval keeps referencing the old one — a classic stale closure bug.",{"fields":212,"seniorities":388,"interview_shapes":389,"locales":394,"oauth":396,"question_count":399,"coach_enabled":400,"jd_match_enabled":400},[213,238,258,275,299,312,331,340,362,369,375,382],{"key":214,"name_tr":215,"name_en":215,"sort":129,"specializations":216},"backend","Backend",[217,220,223,226,229,232,235],{"key":218,"name":219,"field":214},"general","Genel",{"key":221,"name":222,"field":214},"go","Go",{"key":224,"name":225,"field":214},"python","Python",{"key":227,"name":228,"field":214},"java","Java",{"key":230,"name":231,"field":214},"csharp","C#\u002F.NET",{"key":233,"name":234,"field":214},"nodejs","Node.js",{"key":236,"name":237,"field":214},"php","PHP",{"key":239,"name_tr":240,"name_en":240,"sort":173,"specializations":241},"frontend","Frontend",[242,243,246,249,252,255],{"key":218,"name":219,"field":239},{"key":244,"name":245,"field":239},"javascript","JavaScript",{"key":247,"name":248,"field":239},"typescript","TypeScript",{"key":250,"name":251,"field":239},"react","React",{"key":253,"name":254,"field":239},"vue","Vue",{"key":256,"name":257,"field":239},"angular","Angular",{"key":259,"name_tr":260,"name_en":260,"sort":261,"specializations":262},"fullstack","Fullstack",3,[263,264,265,266,267,268,269,270,271,272,273,274],{"key":218,"name":219,"field":259},{"key":221,"name":222,"field":214},{"key":224,"name":225,"field":214},{"key":227,"name":228,"field":214},{"key":230,"name":231,"field":214},{"key":233,"name":234,"field":214},{"key":236,"name":237,"field":214},{"key":244,"name":245,"field":239},{"key":247,"name":248,"field":239},{"key":250,"name":251,"field":239},{"key":253,"name":254,"field":239},{"key":256,"name":257,"field":239},{"key":276,"name_tr":277,"name_en":277,"sort":278,"specializations":279},"devops-cloud","DevOps \u002F Cloud",4,[280,281,284,287,290,293,296],{"key":218,"name":219,"field":276},{"key":282,"name":283,"field":276},"aws","AWS",{"key":285,"name":286,"field":276},"gcp","GCP",{"key":288,"name":289,"field":276},"azure","Azure",{"key":291,"name":292,"field":276},"kubernetes","Kubernetes",{"key":294,"name":295,"field":276},"terraform","Terraform",{"key":297,"name":298,"field":276},"linux","Linux",{"key":300,"name_tr":301,"name_en":301,"sort":302,"specializations":303},"ai-engineer","AI Engineer",5,[304,305,306,309],{"key":218,"name":219,"field":300},{"key":224,"name":225,"field":300},{"key":307,"name":308,"field":300},"llm-rag","LLM\u002FRAG",{"key":310,"name":311,"field":300},"mlops","MLOps",{"key":313,"name_tr":314,"name_en":315,"sort":316,"specializations":317},"database","Veritabanı","Database",6,[318,319,322,325,328],{"key":218,"name":219,"field":313},{"key":320,"name":321,"field":313},"postgresql","PostgreSQL",{"key":323,"name":324,"field":313},"mysql","MySQL",{"key":326,"name":327,"field":313},"mongodb","MongoDB",{"key":329,"name":330,"field":313},"redis","Redis",{"key":5,"name_tr":332,"name_en":6,"sort":333,"specializations":334},"Mobil",7,[335,336,337,338,339],{"key":218,"name":219,"field":5},{"key":121,"name":122,"field":5},{"key":114,"name":115,"field":5},{"key":118,"name":119,"field":5},{"key":124,"name":125,"field":5},{"key":341,"name_tr":342,"name_en":343,"sort":344,"specializations":345},"security","Güvenlik","Security",8,[346,347,350,353,356,359],{"key":218,"name":219,"field":341},{"key":348,"name":349,"field":341},"appsec","AppSec",{"key":351,"name":352,"field":341},"offensive-pentest","Offensive \u002F Pentest",{"key":354,"name":355,"field":341},"cloud-security","Cloud Security",{"key":357,"name":358,"field":341},"devsecops","DevSecOps",{"key":360,"name":361,"field":341},"blue-team-incident","Blue Team \u002F Incident",{"key":363,"name_tr":364,"name_en":365,"sort":366,"specializations":367},"qa-test-automation","QA \u002F Test Otomasyonu","QA \u002F Test Automation",9,[368],{"key":218,"name":219,"field":363},{"key":370,"name_tr":371,"name_en":371,"sort":372,"specializations":373},"data-engineer","Data Engineer",10,[374],{"key":218,"name":219,"field":370},{"key":376,"name_tr":377,"name_en":378,"sort":379,"specializations":380},"game-dev","Oyun Geliştirme","Game Development",11,[381],{"key":218,"name":219,"field":376},{"key":383,"name_tr":384,"name_en":384,"sort":385,"specializations":386},"ml-engineer","ML Engineer",12,[387],{"key":218,"name":219,"field":383},[14,15,16],{"junior":390,"mid":392,"senior":393},{"questions":391,"median_sec":3},20,{"questions":391,"median_sec":3},{"questions":391,"median_sec":3},[395,10],"tr",[397,398],"google","github",21750,true]