this is the code base soo far:
a. Initialize the Project
First, set up a new Node.js project and install necessary dependencies.
mkdir fitness-app-backend
cd fitness-app-backend
npm init -y
npm install express mongoose bcryptjs jsonwebtoken cors dotenv


b. Basic Server Setup (server.js)
// server.js
require('dotenv').config();
const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');

const app = express();

// Middleware
app.use(cors());
app.use(express.json());

// Connect to MongoDB
mongoose.connect(process.env.MONGO_URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
})
.then(() => console.log('MongoDB Connected'))
.catch(err => console.log(err));

// Routes
app.get('/', (req, res) => {
    res.send('Fitness App Backend');
});

// Start Server
const PORT = process.env.PORT || 5000;
app.listen(PORT, () => {
    console.log(`Server running on port ${PORT}`);
});


c. Environment Variables (.env)
Create a .env file in the root directory:
PORT=5000
MONGO_URI=your_mongodb_connection_string
JWT_SECRET=your_jwt_secret_key


d. User Authentication
i. User Model (models/User.js):
// models/User.js
const mongoose = require('mongoose');
const bcrypt = require('bcryptjs');

const UserSchema = new mongoose.Schema({
    username: { type: String, required: true, unique: true },
    email:    { type: String, required: true, unique: true },
    password: { type: String, required: true },
    desiredPhysique: { type: String, enum: ['lean', 'muscular', 'athletic'], required: true },
    fitnessGoals: [String], // e.g., ['strength', 'endurance']
    dietaryPreferences: [String],
    dietaryRestrictions: [String],
}, { timestamps: true });

// Password hashing middleware
UserSchema.pre('save', async function(next) {
    if (!this.isModified('password')) return next();
    try {
        const salt = await bcrypt.genSalt(10);
        this.password = await bcrypt.hash(this.password, salt);
        next();
    } catch(err) {
        next(err);
    }
});

// Password comparison method
UserSchema.methods.comparePassword = function(candidatePassword) {
    return bcrypt.compare(candidatePassword, this.password);
}

module.exports = mongoose.model('User', UserSchema);


ii. Authentication Routes (routes/auth.js):
// routes/auth.js
const express = require('express');
const router = express.Router();
const User = require('../models/User');
const jwt = require('jsonwebtoken');

// Register
router.post('/register', async (req, res) => {
    const { username, email, password, desiredPhysique, fitnessGoals, dietaryPreferences, dietaryRestrictions } = req.body;
    try {
        const newUser = new User({
            username,
            email,
            password,
            desiredPhysique,
            fitnessGoals,
            dietaryPreferences,
            dietaryRestrictions,
        });
        await newUser.save();
        res.status(201).json({ message: 'User registered successfully' });
    } catch(err) {
        res.status(500).json({ error: err.message });
    }
});

// Login
router.post('/login', async (req, res) => {
    const { email, password } = req.body;
    try {
        const user = await User.findOne({ email });
        if (!user) return res.status(400).json({ message: 'Invalid credentials' });

        const isMatch = await user.comparePassword(password);
        if (!isMatch) return res.status(400).json({ message: 'Invalid credentials' });

        const payload = { userId: user._id };
        const token = jwt.sign(payload, process.env.JWT_SECRET, { expiresIn: '1h' });

        res.json({ token });
    } catch(err) {
        res.status(500).json({ error: err.message });
    }
});

module.exports = router;


iii. Protecting Routes (middleware/auth.js):
// middleware/auth.js
const jwt = require('jsonwebtoken');

const auth = (req, res, next) => {
    const token = req.header('Authorization');

    if (!token) return res.status(401).json({ message: 'No token, authorization denied' });

    try {
        const decoded = jwt.verify(token, process.env.JWT_SECRET);
        req.user = decoded.userId;
        next();
    } catch(err) {
        res.status(401).json({ message: 'Token is not valid' });
    }
}

module.exports = auth;


iv. Integrate Routes in server.js:
// server.js (additional lines)
const authRoutes = require('./routes/auth');

app.use('/api/auth', authRoutes);
// Similarly, add other route files like /api/exercises, /api/diets, etc.


4. Implementing Features
Due to the extensive features, I'll provide an example implementation for one feature: Customized Exercise Plans.
a. Exercise Model (models/Exercise.js):
// models/Exercise.js
const mongoose = require('mongoose');

const ExerciseSchema = new mongoose.Schema({
    name: { type: String, required: true },
    description: String,
    musclesTargeted: [String],
    equipment: [String],
    imageUrl: String,
    videoUrl: String,
    difficulty: { type: String, enum: ['beginner', 'intermediate', 'advanced'], default: 'beginner' },
}, { timestamps: true });

module.exports = mongoose.model('Exercise', ExerciseSchema);


b. Exercise Routes (routes/exercises.js):
// routes/exercises.js
const express = require('express');
const router = express.Router();
const Exercise = require('../models/Exercise');
const auth = require('../middleware/auth');

// Get exercises based on user goals
router.get('/', auth, async (req, res) => {
    const userId = req.user;
    try {
        // Fetch user data
        const User = require('../models/User');
        const user = await User.findById(userId);

        // Example logic: Recommend exercises based on desiredPhysique and fitnessGoals
        let query = {};
        if (user.desiredPhysique === 'muscular') {
            query = { musclesTargeted: { $in: ['chest', 'biceps', 'triceps'] }, difficulty: 'intermediate' };
        } else if (user.desiredPhysique === 'lean') {
            query = { musclesTargeted: { $in: ['legs', 'core'] }, difficulty: 'beginner' };
        }
        // Add more logic based on fitnessGoals

        const exercises = await Exercise.find(query).limit(20);
        res.json(exercises);
    } catch(err) {
        res.status(500).json({ error: err.message });
    }
});

// Add new exercise (Admin use)
router.post('/', auth, async (req, res) => {
    // Implement admin authentication if necessary
    const { name, description, musclesTargeted, equipment, imageUrl, videoUrl, difficulty } = req.body;
    try {
        const newExercise = new Exercise({
            name,
            description,
            musclesTargeted,
            equipment,
            imageUrl,
            videoUrl,
            difficulty,
        });
        await newExercise.save();
        res.status(201).json(newExercise);
    } catch(err) {
        res.status(500).json({ error: err.message });
    }
});

module.exports = router;


c. Integrate Exercise Routes in server.js:
// server.js (additional lines)
const exerciseRoutes = require('./routes/exercises');

app.use('/api/exercises', exerciseRoutes);


d. Frontend: Fetching and Displaying Exercises
Assuming you're using React Native with functional components and hooks.
i. Install Dependencies:
yarn add axios
# or
npm install axios


ii. Sample Component (ExerciseList.js):
// components/ExerciseList.js
import React, { useEffect, useState } from 'react';
import { View, Text, FlatList, Image, StyleSheet } from 'react-native';
import axios from 'axios';

const ExerciseList = ({ token }) => {
    const [exercises, setExercises] = useState([]);

    useEffect(() => {
        const fetchExercises = async () => {
            try {
                const res = await axios.get('http://your-backend-url/api/exercises', {
                    headers: {
                        Authorization: token
                    }
                });
                setExercises(res.data);
            } catch (err) {
                console.error(err);
            }
        };
        fetchExercises();
    }, []);

    const renderItem = ({ item }) => (
        <View style={styles.itemContainer}>
            {item.imageUrl && <Image source={{ uri: item.imageUrl }} style={styles.image} />}
            <Text style={styles.title}>{item.name}</Text>
            <Text>{item.description}</Text>
            {item.videoUrl && <Text>Video Tutorial: {item.videoUrl}</Text>}
        </View>
    );

    return (
        <FlatList
            data={exercises}
            keyExtractor={item => item._id}
            renderItem={renderItem}
        />
    );
};

const styles = StyleSheet.create({
    itemContainer: { padding: 10, borderBottomWidth: 1, borderColor: '#ccc' },
    image: { width: '100%', height: 200 },
    title: { fontSize: 18, fontWeight: 'bold' },
});

export default ExerciseList;

how do i do this part?

iii. Using the Component
Ensure you have the user's JWT token after authentication and pass it to the ExerciseList component.
// Example usage in a screen
import React from 'react';
import { View } from 'react-native';
import ExerciseList from '../components/ExerciseList';

const ExercisesScreen = ({ route }) => {
    const { token } = route.params; // assuming token is passed via navigation

    return (
        <View style={{ flex: 1 }}>
            <ExerciseList token={token} />
        </View>
    );
};

export default ExercisesScreen;