[FE] Coding Challenge - Nguyen Van Do

.gitignore

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+node_modules

readme.md

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-# 99Tech Code Challenge #1 #
+# Code Challenge Solutions
 
-Note that if you fork this repository, your responses may be publicly linked to this repo.  
-Please submit your application along with the solutions attached or linked.   
+This repository contains solutions to 3 coding challenges demonstrating proficiency in JavaScript, TypeScript, React, and software engineering best practices.
 
-It is important that you minimally attempt the problems, even if you do not arrive at a working solution.
+---
 
-## Submission ##
-You can either provide a link to an online repository, attach the solution in your application, or whichever method you prefer.
-We're cool as long as we can view your solution without any pain.
+## 📋 Table of Contents
+
+- [Problem 1: Sum to N - Three Ways](#problem-1-sum-to-n---three-ways)
+- [Problem 2: Fancy Form - Currency Swap Application](#problem-2-fancy-form---currency-swap-application)
+- [Problem 3: Messy React - Code Analysis & Refactoring](#problem-3-messy-react---code-analysis--refactoring)
+
+---
+
+## Problem 1: Sum to N - Three Ways
+
+**Location:** [`src/problem1/index.js`](./src/problem1/index.js)
+
+### 📝 Challenge
+Provide 3 unique implementations to compute the sum of numbers from 1 to n.
+
+### ✅ Solutions
+
+| Method | Approach | Time Complexity | Space Complexity |
+|--------|----------|-----------------|------------------|
+| **sum_to_n_a** | Mathematical formula: `n × (n + 1) / 2` | O(1) | O(1) |
+| **sum_to_n_b** | Iterative loop accumulation | O(n) | O(1) |
+| **sum_to_n_c** | Recursive calculation | O(n) | O(n) |
+
+### 🎯 Key Features
+- ✅ Edge case handling (n ≤ 0)
+- ✅ JSDoc documentation
+- ✅ Three distinct algorithmic approaches
+- ✅ Optimal solution using arithmetic series formula
+
+### 💡 Best Practice
+**sum_to_n_a** is the most efficient solution with constant time and space complexity.
+
+---
+
+## Problem 2: Fancy Form - Currency Swap Application
+
+**Location:** [`src/problem2/`](./src/problem2/)
+
+### 📝 Challenge
+Build a responsive, user-friendly currency swap interface with real-time exchange rates.
+
+### 🚀 Live Demo
+https://code-challenge-steel.vercel.app/
+
+### ✨ Features
+
+#### Core Functionality
+-  **Real-time Exchange Rates** - Live price data from CoinGecko API
+-  **Token Swapping** - Swap between USD, ETH, BLUR, bNEO, BUSD, ATOM, LUNA
+-  **Balance Validation** - Prevents swaps exceeding available balance (mocked data for USD and ATOM)
+-  **Auto-calculation** - Instant conversion as you type
+-  **Exchange Rate Display** - Clear rate information
+
+#### User Experience
+-  **Modern Dark UI** - Sleek slate-themed design
+-  **Smooth Animations** - Fade-ins, loading states, transitions
+-  **Fully Responsive** - Mobile, tablet, desktop optimized
+-  **Accessible** - Semantic HTML, ARIA labels, keyboard navigation
+-  **Auto-focus** - Smart input focusing for better UX
+
+#### Technical Features
+-  **Form Validation** - React Hook Form with custom rules
+-  **Type Safety** - Full TypeScript implementation
+-  **Component Architecture** - Modular, reusable components
+-  **Loading States** - Skeleton loaders, spinners
+-  **Error Handling** - Graceful error messages
+-  **Confirmation Modal** - Preview before swap
+
+
+---
+
+## Problem 3: Messy React - Code Analysis & Refactoring
+
+**Location:** [`src/problem3/`](./src/problem3/)
+
+### 📝 Challenge
+Identify computational inefficiencies and anti-patterns in a React TypeScript component, then provide a refactored version following strict best practices.
+
+### 📁 Files
+
+- **[README.md](./src/problem3/README.md)** - Detailed analysis with tables, code comparisons, and explanations
+- **[RefactoredComponent.tsx](./src/problem3/RefactoredComponent.tsx)** - Production-ready implementation
+
+### 🎓 Key Learnings
+
+#### Anti-Patterns Identified
+1. ❌ Using array index as React key
+2. ❌ Memoizing JSX creation with `useMemo`
+3. ❌ Incorrect dependency arrays
+4. ❌ Double iteration over same data
+5. ❌ Functions defined inside component body
+
+#### Correct Patterns
+1. ✅ Use unique, stable identifiers for keys
+2. ✅ Render JSX directly (React optimizes via reconciliation)
+3. ✅ Only memoize expensive calculations
+4. ✅ Single-pass data transformations
+5. ✅ Pure functions outside component or in constants
+
+---
+
+## 🎯 Overall Highlights
+
+### Technical Skills Demonstrated
+- ✅ **Algorithm Design** - Multiple approaches with complexity analysis
+- ✅ **React Expertise** - Hooks, forms, state management, performance optimization
+- ✅ **TypeScript Proficiency** - Strict typing, interfaces, type safety
+- ✅ **Code Quality** - Clean code, documentation, best practices
+- ✅ **Performance** - Optimization, memoization, efficient algorithms
+- ✅ **UX/UI** - Responsive design, accessibility, smooth interactions
+- ✅ **Architecture** - Component design, separation of concerns
+- ✅ **Problem Solving** - Debugging, refactoring, analysis
+
+### Best Practices
+- 📝 Comprehensive documentation
+- 🧪 Type safety throughout
+- ♿ Accessibility compliance
+- 🎨 Modern, responsive UI
+- ⚡ Performance optimization
+- 🔧 Maintainable code structure
+
+---

src/problem1/index.js

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+
+/**
+ * Uses the arithmetic series formula: n × (n + 1) / 2
+ * @param {number} n 
+ * @returns {number}
+ */
+var sum_to_n_a = function (n) {
+    if (n <= 0) return 0;
+    return n * (n + 1) / 2;
+};
+
+/**
+ * Uses a for loop to accumulate the sum.
+ * @param {number} n 
+ * @returns {number}
+ */
+var sum_to_n_b = function (n) {
+    var sum = 0;
+    for (var i = 1; i <= n; i++) {
+        sum += i;
+    }
+    return sum;
+};
+
+/**
+ * Uses recursion to calculate the sum.
+ * @param {number} n 
+ * @returns {number}
+ */
+var sum_to_n_c = function (n) {
+    if (n <= 0) return 0;
+    return n + sum_to_n_c(n - 1);
+};

src/problem2/.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+lerna-debug.log*
+
+node_modules
+dist
+dist-ssr
+*.local
+
+# Editor directories and files
+.vscode/*
+!.vscode/extensions.json
+.idea
+.DS_Store
+*.suo
+*.ntvs*
+*.njsproj
+*.sln
+*.sw?

src/problem2/README.md

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+# React + TypeScript + Vite
+
+This template provides a minimal setup to get React working in Vite with HMR and some ESLint rules.
+
+Currently, two official plugins are available:
+
+- [@vitejs/plugin-react](https://github.com/vitejs/vite-plugin-react/blob/main/packages/plugin-react) uses [Babel](https://babeljs.io/) (or [oxc](https://oxc.rs) when used in [rolldown-vite](https://vite.dev/guide/rolldown)) for Fast Refresh
+- [@vitejs/plugin-react-swc](https://github.com/vitejs/vite-plugin-react/blob/main/packages/plugin-react-swc) uses [SWC](https://swc.rs/) for Fast Refresh
+
+## React Compiler
+
+The React Compiler is not enabled on this template because of its impact on dev & build performances. To add it, see [this documentation](https://react.dev/learn/react-compiler/installation).
+
+## Expanding the ESLint configuration
+
+If you are developing a production application, we recommend updating the configuration to enable type-aware lint rules:
+
+```js
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      // Other configs...
+
+      // Remove tseslint.configs.recommended and replace with this
+      tseslint.configs.recommendedTypeChecked,
+      // Alternatively, use this for stricter rules
+      tseslint.configs.strictTypeChecked,
+      // Optionally, add this for stylistic rules
+      tseslint.configs.stylisticTypeChecked,
+
+      // Other configs...
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.node.json', './tsconfig.app.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+      // other options...
+    },
+  },
+])
+```
+
+You can also install [eslint-plugin-react-x](https://github.com/Rel1cx/eslint-react/tree/main/packages/plugins/eslint-plugin-react-x) and [eslint-plugin-react-dom](https://github.com/Rel1cx/eslint-react/tree/main/packages/plugins/eslint-plugin-react-dom) for React-specific lint rules:
+
+```js
+// eslint.config.js
+import reactX from 'eslint-plugin-react-x'
+import reactDom from 'eslint-plugin-react-dom'
+
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      // Other configs...
+      // Enable lint rules for React
+      reactX.configs['recommended-typescript'],
+      // Enable lint rules for React DOM
+      reactDom.configs.recommended,
+    ],
+    languageOptions: {
+      parserOptions: {
+        project: ['./tsconfig.node.json', './tsconfig.app.json'],
+        tsconfigRootDir: import.meta.dirname,
+      },
+      // other options...
+    },
+  },
+])
+```

src/problem2/eslint.config.js

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+import js from '@eslint/js'
+import globals from 'globals'
+import reactHooks from 'eslint-plugin-react-hooks'
+import reactRefresh from 'eslint-plugin-react-refresh'
+import tseslint from 'typescript-eslint'
+import { defineConfig, globalIgnores } from 'eslint/config'
+
+export default defineConfig([
+  globalIgnores(['dist']),
+  {
+    files: ['**/*.{ts,tsx}'],
+    extends: [
+      js.configs.recommended,
+      tseslint.configs.recommended,
+      reactHooks.configs.flat.recommended,
+      reactRefresh.configs.vite,
+    ],
+    languageOptions: {
+      ecmaVersion: 2020,
+      globals: globals.browser,
+    },
+  },
+])

src/problem2/index.html

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-<html>
+<!doctype html>
+<html lang="en">
 
 <head>
-  <meta name="viewport" content="width=device-width, initial-scale=1">
-  <title>Fancy Form</title>
-
-  <!-- You may add more stuff here -->
-  <link href="style.css" rel="stylesheet" />
+  <meta charset="UTF-8" />
+  <link rel="icon" type="image/svg+xml" href="/vite.svg" />
+  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+  <meta name="description" content="Swap cryptocurrency assets instantly with real-time exchange rates" />
+  <title>Currency Swap - Exchange Crypto Assets</title>
 </head>
 
 <body>
-
-  <!-- You may reorganise the whole HTML, as long as your form achieves the same effect. -->
-  <form onsubmit="return !1">
-    <h5>Swap</h5>
-    <label for="input-amount">Amount to send</label>
-    <input id="input-amount" />
-
-    <label for="output-amount">Amount to receive</label>
-    <input id="output-amount" />
-
-    <button>CONFIRM SWAP</button>
-  </form>
-  <script src="script.js"></script>
+  <div id="root"></div>
+  <script type="module" src="/src/main.tsx"></script>
 </body>
 
-</html>
+</html>

src/problem2/package.json

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+{
+  "name": "problem2",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "lint": "eslint .",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@tailwindcss/vite": "^4.1.18",
+    "react": "^19.2.0",
+    "react-dom": "^19.2.0",
+    "react-hook-form": "^7.68.0",
+    "tailwindcss": "^4.1.18"
+  },
+  "devDependencies": {
+    "@eslint/js": "^9.39.1",
+    "@types/node": "^24.10.1",
+    "@types/react": "^19.2.5",
+    "@types/react-dom": "^19.2.3",
+    "@vitejs/plugin-react": "^5.1.1",
+    "eslint": "^9.39.1",
+    "eslint-plugin-react-hooks": "^7.0.1",
+    "eslint-plugin-react-refresh": "^0.4.24",
+    "globals": "^16.5.0",
+    "typescript": "~5.9.3",
+    "typescript-eslint": "^8.46.4",
+    "vite": "^7.2.4"
+  }
+}