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const data = [
  {
    id: 251,
    name: 'Top level 1',
    parentId: null,
    children: [
      {
        id: 253,
        parentId: 251,
        name: 'Inner level 1-1',

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// Basic usage of reduce to find sum of numbers in an array
const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce((prev, curr) => prev + curr, 0);
console.log('sum', sum);

// Basic usage of reduce to convert an array of objects into an object.
// For e.g.
// Input -> [{ id: 1, key: 'a', value: 'a' }, { id: 2, key: 'b', value: 'b' }, { id: 3, key: 'c', value: 'c' }];
// Output -> { 1: { key: 'a', value: 'a' }, 2: { key: 'b', value: 'b' }, 3: { key: 'c', value: 'c' } };
// Now you might ask, what's the use of this and where we might use this?

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Guidelines

General Guidelines

• Write appropriate unit tests for your code. Try to cover 100% of your code. Write tests for positive as well negative outcomes. Take edge cases into account.
• If the problem is in Typescript, use default and custom types diligently. Please do not use any type. We care about type safety and you should too.
• Make sure your code is well written and follows proper coding conventions.
  • Add useful comments wherever required
  • Lint your code
• Use user-friendly, readable variable names etc.

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Phases

1. Initialisation
2. Mounting
3. Updating
4. Unmounting

1. Initialisation
Setting up of initial state and default props happen here.
  a. constructor

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What is [[Prototype]]?
  Every object has a special hidden property called [[Prototype]] which is used to access that object's prototype.
  
  prototype object <- [[Prototype]] <- object
  
  [[Prototype]] is hidden and internal
  
  __proto__ can be used to set [[Prototype]]
  Fun fact:
    1. __proto__ is a historical getter/setter for [[Prototype]]

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function explode(array, chunkSize) {
    if (chunkSize === 0) return [];
    const explodedArr = [];
    let innerArr = [];
    for (let i = 0; i < array.length; i++) {
        innerArr.push(array[i]);
        if ( (i + 1) % chunkSize === 0) {
            explodedArr.push(innerArr);
            innerArr = [];
        }

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import { Injectable } from '@angular/core';
import { HttpEvent, HttpHandler, HttpInterceptor, HttpRequest, HttpResponse } from '@angular/common/http';
import { Observable, timer, throwError, of } from 'rxjs';
import { retryWhen, tap, mergeMap } from 'rxjs/operators';


@Injectable()
export class HttpRequestInterceptor implements HttpInterceptor {
  retryDelay = 2000;
  retryMaxAttempts = 2;

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import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/http';
import { Observable } from 'rxjs';
import { concatMap } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class TestService {
  constructor(private readonly _http: HttpClient) {}

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import React, { Component } from 'react';

class FormTest extends Component {
    constructor(props) {
        super(props);
        this.state = {
            username: {
                value: '',
                valid: false,
                errorMessage: ''

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Lifecycle hooks Angular

constructor
ngOnChanges
ngOnInit
ngDoCheck
    ngAfterContentInit
    ngAfterContentChecked
    ngAfterViewInit
    ngAfterViewChecked