groupBy(arr, key)

Write a function groupBy that takes an array of objects and a property name (as a string), and returns an object where the keys are the unique values of the specified property, and the values are arrays of objects that have that property value.

Example Inputs & Outputs

// Example 1:
const users = [
    { name: 'Alice', age: 25 },
    { name: 'Bob', age: 30 },
    { name: 'Charlie', age: 25 }
];
groupBy(users, 'age');
// Output: {
//     '25': [{ name: 'Alice', age: 25 }, { name: 'Charlie', age: 25 }],
//     '30': [{ name: 'Bob', age: 30 }]
// }

// Example 2:
const products = [
    { id: 1, category: 'Electronics' },
    { id: 2, category: 'Clothing' },
    { id: 3, category: 'Electronics' }
];
groupBy(products, 'category');
// Output: {
//     'Electronics': [{ id: 1, category: 'Electronics' }, { id: 3, category: 'Electronics' }],
//     'Clothing': [{ id: 2, category: 'Clothing' }]
// }

Constraints & Edge Cases

• The input array may be empty, in this case return an empty object.
• The key values can be of any type (number, string, etc.), but they will be coerced to strings when used as object keys.