Workout Routine Generator JavaScript, API

```javascript
// This code provides a basic framework.  You'll need to:
// 1.  Choose and integrate a suitable workout API (e.g., ExerciseDB, MuscleWiki API - many are free but may require registration and API keys).
// 2.  Implement error handling and input validation.
// 3.  Consider data persistence (local storage, databases) for user preferences.
// 4.  Add more sophisticated logic for exercise selection (equipment, experience level).

// Placeholder API URL - REPLACE WITH AN ACTUAL API URL
const API_URL = "https://example.com/api/exercises";  // Replace this!!

async function fetchExercises(muscleGroup = null, type = null) {
  let url = API_URL;
  if (muscleGroup) {
    url += `?muscle=${muscleGroup}`; // Example query parameter
  }
  if (type) {
      url += `&type=${type}`; // Added type query param
  }

  try {
    const response = await fetch(url);
    if (!response.ok) {
      throw new Error(`HTTP error! Status: ${response.status}`);
    }
    const data = await response.json();
    return data;
  } catch (error) {
    console.error("Error fetching exercises:", error);
    return []; // Return an empty array or handle the error as needed
  }
}


function generateWorkout(exercises, numExercises = 5) {
  if (!exercises || exercises.length === 0) {
    return "No exercises available. Please try different criteria.";
  }

  const workout = [];
  const shuffledExercises = [...exercises].sort(() => Math.random() - 0.5); // Shuffle

  for (let i = 0; i < Math.min(numExercises, shuffledExercises.length); i++) {
    workout.push(shuffledExercises[i]);
  }

  if (workout.length === 0) {
      return "No exercises matched the criteria.";
  }

  let workoutString = "Your Workout:\n";
  workout.forEach((exercise, index) => {
    workoutString += `${index + 1}. ${exercise.name} - ${exercise.type} (Target: ${exercise.muscle})\n`; // Adjust based on API response format
  });

  return workoutString;
}


// Example usage (you'll need to adapt this for user interaction)
async function main() {
  const muscleGroup = prompt("Enter a muscle group (e.g., chest, back, legs) or leave blank for all:");
  const exerciseType = prompt("Enter a type of exercise (e.g. cardio, strength) or leave blank for all:");

  const exercises = await fetchExercises(muscleGroup, exerciseType);
  const workout = generateWorkout(exercises, 5); //Generate 5 exercises
  console.log(workout);

  // Display the workout in a user-friendly way (e.g., in an HTML element).
  //document.getElementById("workout-output").innerText = workout; // Example if you're using HTML
}

main();

// Example Data Structure (mimicking a typical API response)
// Adapt based on the actual API you use!
/*
[
  {
    "id": 1,
    "name": "Bench Press",
    "type": "strength",
    "muscle": "chest",
    "equipment": "barbell",
    "difficulty": "intermediate",
    "instructions": "Lie on a bench and press the barbell up."
  },
  {
    "id": 2,
    "name": "Squats",
    "type": "strength",
    "muscle": "legs",
    "equipment": "barbell",
    "difficulty": "intermediate",
    "instructions": "Stand with the barbell across your upper back and squat down."
  },
  {
    "id": 3,
    "name": "Running",
    "type": "cardio",
    "muscle": "legs",
    "equipment": "none",
    "difficulty": "beginner",
    "instructions": "Run at a moderate pace."
  }

]
*/
```