Object (JavaScript)

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Iterable ProtocolValueDestructuringWeakSetWeakMap

Object

A value constructed with new unless using array, object, or regexp literal notation. Objects are always referred to by variables with a reference.

Remember

keys are converted to strings
if (obj.foo) checks truthiness, not membership
reading a missing key evaluates to undefined
when possible, obj.foo is preferred to obj['foo']
similarly, { a: 1 } is preferred to { 'a': 1 }

Construction

const obj = {};
obj;

{}

const obj = {
  foo: 'hello',
  bar: 'world'
};
obj;

{ foo: 'hello', bar: 'world' }

Get

const object = { foo: 1 };
object.foo;

const key = 'foo';
object[key];

object.missing;

undefined

Set

Keys are strings

Keys are converted to strings.

const object = {};
object[1] = 'number';
object['1'] = 'string';
object;

{ '1': 'string' }

const object = {};
object.a = 1;
object['b'] = 2;
object;

{ a: 1, b: 2 }

Remove

const object = { a: 1, b: 2 };
delete object.a;
object;

{ b: 2 }

delete object['b'];

{}

Check Membership

Falsey values vs key existence

Do you want to check if an object has a key, or that key's value is truthy?

const obj = { foo: 0 };
if (obj.foo) console.log('will not run');
if ('foo' in obj) console.log('will run');

'will run'

const object = { foo: 'bar' };
'foo' in object

true

object.hasOwnProperty('foo');

true

Iterating

const object = {
  a: 'one',
  b: 'two',
  c: 'three'
};
Object.keys(object);

['a', 'b', 'c']

Object.values(object);

['one', 'two', 'three']

Object.entries(object);

[
  ['a', 'one'],
  ['b', 'two'],
  ['c', 'three']
]

for (const key of Object.keys(object)) {
  console.log(key);
}

'a'
'b'
'c'

Examples

const foodPrices = {
  taco: 3,
  burrito: 8
};
const prices = {
  ...foodPrices,
  water: 20
};
prices.water = 0;

function getPrice(item) {
  if (item in prices) {
    return `we do not sell ${item}`;
  } else {
    return `${item}: ${prices[item]} dollars`;
  }
}

getPrice('taco');

'taco: 3 dollars'

getPrice('water');

'water: 0 dollars'

getPrice('fries');

'we do not sell fries'

const arr = [1, '1', 2];
cosnt counts = {};

for (const el of arr) {
  counts[el] = (counts[el] || 0) + 1;
}

counts;

{ '1': 2, '2': 1 }

const object = {
  
};