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A Practical Guide to Object Destructuring in JavaScript

What is object destructuring?

In short, object destructuring is a syntax that allows you to extract values from an object and assign them to variable names.

const favoriteFruit = {
  name: "apple",
  color: "red",
};

const name = favoriteFruit.name;
const color = favoriteFruit.color;

// is equivalent to
const { name, color } = favoriteFruit;

How I like to think about it is that we’re stripping the properties we need from the object. But aside from being slightly more consise, why is destructuring useful? For that we need a more practical example.

A real world example

const item = {
  id: 1234,
  name: "Metal Bottle",
  variation: "red",
  price: {
    currency: "NZD",
    amount: 15.0,
    tax: 0.15,
  },
};

Here we have an item object that has a bunch of properties. Say we need to define a function that takes the id and variation properties from the item and return the stock numbers.

function getStock(item) {
  const id = item.id;
  const variation = item.variation;
  // make a fetch request to some API using the id and variation

  return stock;
}

getStock(item);

Here’s an example of how that would be accomplished without object destructuring. To contrast, here’s how the same functionality would be accomplished with object destructuring.

function getStock({ id, variation }) {
  // make a fetch request to some API using the id and variation

  return stock;
}

getStock(item);

What this does is take the item object we’ve passed in, finds the id and variation properties on it and assigns the values to the two variables.

There are a few benefits to this approach:

  1. The properties used in the function are made obvious. Looking at the function, you can see that it only uses the id and variation properties.
  2. Primitive properties are copied when you use destructuring, which helps to prevent side effects.
  3. VSCode (and probably other code editors) autocompletion can help you when calling the function.

Code suggestions from VS Code.
Code suggestions from VS Code.

Number two might be a little confusing to newer programmers and those unfamilliar with JavaScript. Firstly, what is a primitive? A primitive in JavaScript is defined as data that is not an object and has no methods. This includes strings, numbers and booleans.

Another term that may be unfamilliar is side effects. A function with side effects is one that uses or changes a variable outside it’s scope. Or put simply, what you pass as the function’s arguments is all it should interact with. For example:

console.log(item.name); // "Metal Bottle"

function prepareItemForCheckout(item) {
  // Processes the item data for checkout.
  item.name = `${item.name} (${item.variation})`;
  return;
}

prepareItemForCheckout(item);

console.log(item.name); // "Metal Bottle (red)"

As you can see, we’re modifying the item’s name inside the function, which affects the item variable in the global scope. Which could end up breaking your UI rendering logic. In prepareItemForCheckout I could have changed the name property to a list, and maybe somewhere else in my code, it relies on name to have the .toUpper() method.

Lets take a look at a more complex example.

const item = {
  id: 1234,
  name: "Metal Bottle",
  variation: "red",
  price: {
    currency: "NZD",
    amount: 15.0,
    tax: 0.15,
  },
};

function computeTotalPrice({
  item: {
    price: { amount, tax },
  },
  firstTimeDiscount = false,
}) {
  const priceWithTax = amount * (1 + tax);

  if (firstTimeDiscount) {
    return priceWithTax * 0.9;
  }

  return priceWithTax;
}

console.log(computeTotalPrice({ item })); // 17.25
console.log(computeTotalPrice({ item, firstTimeDiscount: true })); // 15.525

Here we have a function that uses object destructuring with an optional argument, firstTimeDiscount set to false by default. It finds the item property on the object, and finds the price property on it. Finally, it declares the variables amount and tax. Note that you can’t access item and price as variables within the local scope.

I’d like to draw attention to the function call. What would it look like without object destructuring?

// with object destructuring
computeTotalPrice({ item, firstTimeDiscount: true });
// without
computeTotalPrice(item, true);

Sorry to get philosophical, but what is true? It’s impossible to tell how that argument is being used by the function without looking at the function definition itself. With object destructuring, the purpose is crystal clear.

Conclusion

To reiterate, object destructuring in JavaScript is useful because it:

  1. Makes the properties used in a function more obvious.
  2. Makes function calls more readable by providing context such as firstTimeDiscount: true
  3. Helps to prevent function side effects.
  4. Enables some code editors to give more helpful suggestions.

Written on in tags #javascript #tutorial