About

Object destructuring is a concise way of extracting properties from an object. Its syntax is similar to an object literal expression, but on the left-hand side of the assignment instead of the right.

const weather = {
  sun: '☀️',
  sun_behind_small_cloud: '🌤️',
  sun_behind_cloud: '⛅',
  sun_behind_large_cloud: '🌥️',
  sun_behind_rain_cloud: '🌦️',
  cloud: '☁️',
  cloud_with_rain: '🌧️',
  cloud_with_snow: '🌨️',
  cloud_with_lightning: '🌩️',
  cloud_with_lightning_and_rain: '⛈️',
};

const { sun, cloud, cloud_with_lightning } = weather;

sun;
// => '☀️'

cloud;
// => '☁️'

cloud_with_lightning;
// => '🌩️'

Renaming in assignment

The syntax can extract the properties by their key like sun, cloud, and cloud_with_lightning, but can also pick a different name for the variable:

const { sun: sunny, cloud: cloudy, cloud_with_rain: rainy } = weather;

typeof cloud_with_rain;
// => 'undefined'

typeof rainy;
// => 'string'

rainy;
// => 🌧️

The assignment is also not required to use all the values.

Default values

The object destructuring assignment can provide default values in case there is none in the source object:

const { sun = '🌞', tornado = '🌪️', cloud_with_snow: snowy = '❄️' } = weather;

sun;
// => '☀️'

tornado;
// => '🌪️'

snowy;
// => '🌨️'

The following is observed:

• sun has extracted from the object weather without replacing it as it is present in the object weather,
• tornado does not exist in the object weather, so the default value was used,
• cloud_with_snow was extracted as the variable snowy, without replacing it, as cloud_with_snow is present in the object weather.

Destructuring function parameters

The weather object has a lot of properties. It is possible to directly extract one or multiple properties from this object when it’s passed to a function:

function weatherReport({ sun }) {
  console.log(sun);
}

weatherReport(sun);
// => '☀️'

Destructuring for of iteration

When iterating over an array (or other iterable), and the items are objects, it is possible to destructure inside the for (...) of iterable statement:

const { sun: sunny, cloud: cloudy, cloud_with_rain: rainy } = weather;

const prediction = [
  {
    chance: 0.8,
    weather: sunny,
    description: 'There is a 80% chance it will remain sunny.',
  },
  {
    chance: 0.15,
    weather: cloudy,
    description:
      'There is a 15% chance the clouds will keep the sun from poking through.',
  },
  {
    chance: 0.05,
    weather: rainy,
    description: 'There is a small chance of rain.',
  },
];

for (const { weather: symbol, description } of prediction) {
  console.log(`${symbol}: ${description}`);
}

//  '☀️: There is a 80% chance it will remain sunny.'
// '☁️: There is a 15% chance the clouds will keep the sun from poking through.'
// '🌧️: There is a small chance of rain.'

Related concepts

concept:javascript/array-destructuring concept:javascript/rest-and-spread

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Originally from Exercism javascript concepts