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The Object.entries() method, introduced in ES8 (ECMAScript 2017), returns an array of a given object's own enumerable string-keyed property [key, value] pairs. This is incredibly useful for iterating over object properties or for converting objects into a Map.

Example 1: Basic Usage

const person = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30
};

// Using Object.entries() to get an array of [key, value] pairs
const entries = Object.entries(person);

console.log(entries);
// Expected output: [ ['firstName', 'John'], ['lastName', 'Doe'], ['age', 30] ]

Explanation

This example demonstrates the fundamental use of Object.entries(). It takes the person object and returns an array where each inner array contains a key and its corresponding value.

Example 2: Iterating with for...of loop

const user = {
  username: 'coder123',
  email: 'coder123@example.com',
  isLoggedIn: true
};

// Looping through the entries of the user object
for (const [key, value] of Object.entries(user)) {
  console.log(`Key: ${key}, Value: ${value}`);
}

Explanation

Here, Object.entries() is used within a for...of loop to conveniently access both the key and value of each property in the user object. This is a common and readable way to iterate over object data.

Example 3: Converting an Object to a Map

const car = {
  make: 'Honda',
  model: 'Civic',
  year: 2022
};

// Creating a new Map from the car object
const carMap = new Map(Object.entries(car));

console.log(carMap.get('model')); // Expected output: Civic

Explanation

This code shows how Object.entries() can be used to convert an object into a Map. The Map constructor accepts an iterable of [key, value] pairs, which is exactly what Object.entries() provides.

Example 4: Filtering Object Properties

const product = {
  name: 'Laptop',
  price: 1200,
  inStock: true,
  rating: 4.5
};

// Filtering for properties where the value is a number
const numericProperties = Object.entries(product).filter(([key, value]) => typeof value === 'number');

console.log(numericProperties);
// Expected output: [ ['price', 1200], ['rating', 4.5] ]

Explanation

This example demonstrates how to filter the properties of an object. Object.entries() creates the array, and then the filter method is used to create a new array containing only the properties with numeric values.

Example 5: Handling Objects with symbol properties

const id = Symbol('id');
const employee = {
  name: 'Jane',
  [id]: 12345
};

// Object.entries() only includes string-keyed properties
const employeeEntries = Object.entries(employee);

console.log(employeeEntries); // Expected output: [ ['name', 'Jane'] ]

Explanation

This code illustrates that Object.entries() ignores properties with Symbol keys. Only the name property, which has a string key, is included in the resulting array.

Example 6: Using with Array-like Objects

const arrayLike = {
  0: 'a',
  1: 'b',
  2: 'c'
};

// Using Object.entries() on an array-like object
const entries = Object.entries(arrayLike);

console.log(entries);
// Expected output: [ ['0', 'a'], ['1', 'b'], ['2', 'c'] ]

Explanation

This example shows that Object.entries() works with array-like objects. The numeric keys are converted to strings in the resulting array of [key, value] pairs.

Example 7: Transforming Keys to Uppercase

const settings = {
  theme: 'dark',
  fontSize: 16,
  notifications: true
};

// Creating a new object with uppercase keys
const upperCaseSettings = Object.fromEntries(
  Object.entries(settings).map(([key, value]) => [key.toUpperCase(), value])
);

console.log(upperCaseSettings);
// Expected output: { THEME: 'dark', FONTSIZE: 16, NOTIFICATIONS: true }

Explanation

This example combines Object.entries() with map and Object.fromEntries() to transform an object. It gets the [key, value] pairs, converts each key to uppercase, and then creates a new object from the transformed pairs.

JavaScript

Object.entries()