Destructuring Assignment

Learning Goals

Use destructuring to assign data to variables

Introduction

As developers, sometimes we receive information in a collection like an Object and we want to "pick and choose" elements out of the collection. It's a major pain to extract each property / value pair out of an Object and then assign it to a variable.

Destructuring lets us type less and be more clear about what we want to pull out of an Object. Not only does destructuring help when working with data in your application, it's essential for understanding how to get JavaScript to include third-party code (like you find on npm).

Use Destructuring to Assign Data to Variables

In JavaScript, when we want to assign data to single variables from a top level object such as this, we normally do it individually like so:

const doggie = {
  first: 'Buzz',
  breed: 'Great Pyrenees',
  fur_color: 'black and white',
  activity_level: 'sloth-like',
  favorite_food: 'hot_dogs'
};
const first = doggie.first;  // Buzz
const breed = doggie.breed; // Great Pyrenees

This is repetitive code. The algorithm at play is:

Declare a variable with a name (e.g. first or breed)
Use that variable's name to point to an attribute in the Object whose name matches the name of the variable (e.g. doggie.breed or doggie.first)
Assign the attribute's value to the created variable

JavaScript gives us the ability to perform this algorithm with one simple line of code.

const doggie = {
  first: 'Buzz',
  breed: 'Great Pyrenees',
  fur_color: 'black and white',
  activity_level: 'sloth-like',
  favorite_food: 'hot_dogs'
};

const { first, breed } = doggie;
console.log(first); 
console.log(breed);

We can also use it to destructure a nested syntax:

const doggie = {
  first: 'Buzz',
  breed: 'Great Pyrenees',
  fur_color: 'black and white',
  activity_level: 'sloth-like',
  favorite_foods: {
    meats:{
      ham: 'smoked',
      hot_dog: 'oscar_meyer',
    },
    cheeses:{
      american: 'kraft'
    }
  }
};

const { ham, hot_dog } = doggie.favorite_foods.meats;
console.log(ham); 
console.log(hot_dog);

Destructuring Assignment with Arrays

Destructuring does not just work on objects - we can also use the same syntax with Arrays as well.

const dogs = ['Great Pyrenees', 'Pug', 'Bull Mastiff']
const [medium, small, giant] = dogs
console.log(medium, small, giant) // Great Pyrenees, Pug, Bull Mastiff

The cool part is we can pick the parts of the Array that we want to assign!

const dogs = ['Great Pyrenees', 'Pug', 'Bull Mastiff']
const [, small, giant] = dogs
console.log(small, giant) //  Pug, Bull Mastiff

Destructuring Assignment with Strings

We can also destructure with strings, as a whole:

const dogsName = 'Sir Woody BarksALot'
const [title, firstName, lastName] = 'Sir Woody BarksALot'.split(' ')
console.log(title, firstName, lastName) // Sir Woody BarksALot

And we can also destructure it in parts, just as we did with Arrays above:

const dogsName = 'Sir Woody BarksALot'
const [title, ,lastName] = 'Sir Woody BarksALot'.split(' ')
console.log(title, lastName) // Sir BarksALot

Instructions

We are going to give you several Strings, Arrays, and Objects and you're going to write several destructuring assignments for each. Write your code in the index.js file. Let the instructions in the README and the tests guide you through the process.

Conclusion

"Destructuring assignment" is a fast, and efficient way to assign data to variables from objects, Arrays, and strings. It allows us to pick and choose the pieces of data that we want to assign, and gives us lots of freedom to manipulate the data as it is coming in. With practice, you'll be proficient at it in no time.

Resources

Destructuring assignment

mkirby / fewpjs_destructuring_assignment