Objects

You create an object using curly brackets. You can also directly include some entries. For that, state the key first, followed by a colon and the value.

const emptyObject = {};

const obj = {
  favoriteNumber: 42,
  greeting: 'Hello World',
  useGreeting: true,
  address: {
    street: 'Trincomalee Highway',
    city: 'Batticaloa',
  },
  fruits: ['melon', 'papaya'],
  addNumbers: function (a, b) {
    return a + b;
  },
};

The trailing comma after the last entry is optional in JavaScript.

If the key follows the naming rules for a JavaScript identifier, you can omit the quotation marks. For keys with special characters in the name, you need to use the usual string notation.

const obj = {
  '1keyStartsWithNumber': '...',
  'key/with/slashes': '...',
  'key-with-dashes': '...',
  'key with spaces': '...',
  '#&()[]{}èä樹keyWithSpecialChars': '...',
};

There are two ways to retrieve the value for a given key, dot notation and bracket notation.

const obj = { greeting: 'hello world' };

obj.greeting;
// => hello world

obj['greeting'];
// => hello world

// Bracket notation also works with variables.
const key = 'greeting';
obj[key];
// => hello world

Using the dot notation as a shorthand has the same restriction as omitting the quotation marks. It only works if the key follows the identifier naming rules.

You can add or change a value using the assignment operator =. Again, there are dot and bracket notations available.

const obj = { greeting: 'hello world' };

obj.greeting = 'Hi there!';
obj['greeting'] = 'Welcome.';

obj.newKey1 = 'new value 1';
obj['new key 2'] = 'new value 2';

const key = 'new key 3';
obj[key] = 'new value 3';

You can delete a key-value pair from an object using the delete keyword.

const obj = {
  key1: 'value1',
  key2: 'value2',
};

delete obj.key1;
delete obj['key2'];

You can check whether a certain key exists in an object with the hasOwnProperty method.

const obj = { greeting: 'hello world' };

obj.hasOwnProperty('greeting');
// => true

obj.hasOwnProperty('age');
// => false

There is a special for...in loop to iterate over all keys of an object.

const obj = {
  name: 'Ali',
  age: 65,
};

for (let key in obj) {
  console.log(key, obj[key]);
}
// name Ali
// age 65

To avoid subtle errors you should always assume the for...in loop visits the keys in an arbitrary order. Also, be aware that for...in includes inherited keys in its iteration.