Frontend

const createCounter = () => {

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import React, { useState, useEffect, useRef, useContext, useReducer, useMemo } from 'react'; export default Effects = () => { const [count, setCount] = useState(0); useLayoutEffect(() => console.log("each render 1")); console.log("each render 2"); useEffect(() => console.log("each render 3")); return ( <> <button onClick={() => setCount((prevProps) => ++prevProps)}> Increment </button> </> ) }

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What are the scopes of let/const and var? Is there another way to declare variables besides var, let, const?

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What cookie settings are available (httpOnly, secure, expires, domain, path)?

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How do you understand what rendering in React is? What can cause it to happen?

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const func = obj.sayHi() const boundFuns = func.bind(obj) boundFuns() ------------------- const submit = (el) => () => { log(el); } arr.map(el => <button onClick={submit(el)}/>) const param = useMemo(() => a + b) return <Child value={param} />

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Have you ever worked without an analyst, when you had to figure out the task on your own?

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What is WeakMap?

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const { foo: bar, bar: foo } = { foo: 1, bar: 2 } console.log(bar) // console.log(foo) //

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const { foo: bar, bar: foo } = { foo: 1, bar: 2 }; //foo //bar setTimeout(() => { console.log("timeOut"); }, 0); console.log(1); new Promise((resolve) => { console.log("Promise"); setTimeout(() => { console.log("timeOut2"); resolve(); }, 0); }) .then(() => { console.log("then1"); }) .then(() => { console.log("then2"); }); console.log(4); setTimeout(() => { console.log("timeOut3"); }, 0);

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How does React compare props? How does shallow comparison work?

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Why is useRef needed? What are its applications?

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If the parent component passes a new array as props, will you see the new data on the screen or the old? Why?

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Type the getParam function so that TypeScript automatically highlights the presence of properties in the object. Make the typing so that arrays cannot be passed.

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let obj = { a: 1 }; let array = [obj]; obj = null; console.log(obj); // console.log(array); //

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import { useState, useRef, useEffect } from "react"; const useDebugRender = ({props}) => { const prevProp = useRef(props) useEffect(() => { }, []) } const ParentComponent = () => { const [random, setRandom] = useState(0); // Passes to child component as a Prop const [text, setText] = useState(""); // Passes to child component as a Prop const createRandom = () => setRandom(Math.floor(Math.random() * 100)); const onTextChange = (e) => setText(e.target.value); const [count, setCount] = useState(0); const incrementCount = () => setCount((prev) => prev + 1); // This function is passed as a prop to the child component return ( <> <Count: {count} /> <input type="text" onChange={onTextChange} /> <button onClick={createRandom}>Generate Random</button> <div> <ChildComponent random={random} text={text} incrementCount={incrementCount} /> </div> </> ); }; export default ParentComponent;

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const Effects = () => { const [count, setCount] = useState(0); useLayoutEffect(() => console.log("each render 1")); console.log("each render 2"); useEffect(() => console.log("each render 3")); return ( <> {count} <button onClick={() => setCount((prevProps) => ++prevProps)}> Increment </button> </> ); }; export default Effects; // 1 рендер: // После клика: const Effects = () => { const [count, setCount] = useState(0); const [number, setNumber] = useState(0); useEffect(() => { console.log(count); return () => { console.log(count); }; }); return ( <> {count} <button onClick={() => setCount((prevProps) => ++prevProps)}> Increment </button> </> ); }; export default Effects; // 1 рендер: // 2 1 3 // После клика: // 2 1 3 // return null: // 2 1 3 // // 1: 1, 0 // +: 1, 2 // ++: 2, 2

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What is the difference between sequential await p1, await p2 and Promise.all([p1, p2])?

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const submit = useCallback((el) => () => { log(el); }, []); arr.map(el => <button onClick={submit(el)} />) arr.map(el => <SomeComp key={} onClick={() => submit(el)} />) const param = useMemo(() => ({c: a + b}), [a,b]) return <Child value={param} /> Child = React.memo(() => {})

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Do let and const (hoisting) pop up? What happens when you access them before declaration?

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