JavaScript Object Destructuring

Summary: in this tutorial, you’ll learn about JavaScript object destructuring which assigns properties of an object to individual variables.

If you want to learn how to destructure an array, you can check out the array destructuring tutorial.

Introduction to the JavaScript object destructuring assignment

Suppose you have a person object with two properties: firstName and lastName.

let person = {
    firstName: 'John',
    lastName: 'Doe'
};Code language: JavaScript (javascript)

Before ES6, when you want to assign properties of the person object to variables, you typically do it like this:

let firstName = person.firstName;
let lastName = person.lastName; Code language: JavaScript (javascript)

ES6 introduces the object destructuring syntax that provides an alternative way to assign properties of an object to variables:

let { firstName: fname, lastName: lname } = person;Code language: JavaScript (javascript)

In this example, the firstName and lastName properties are assigned to the fName and lName variables respectively.

In this syntax:

let { property1: variable1, property2: variable2 } = object;Code language: JavaScript (javascript)

The identifier before the colon (:) is the property of the object and the identifier after the colon is the variable.

Notice that the property name is always on the left whether it’s an object literal or object destructuring syntax.

If the variables have the same names as the properties of the object, you can make the code more concise as follows:

let { firstName, lastName } = person;

console.log(firstName); // 'John'
console.log(lastName); // 'Doe'Code language: JavaScript (javascript)

In this example, we declared two variables firstName and lastName, and assigned the properties of the person object to the variables in the same statement.

It’s possible to separate the declaration and assignment. However, you must surround the variables in parentheses:

({firstName, lastName} = person);

If you don’t use the parentheses, the JavaScript engine will interpret the left-hand side as a block and throw a syntax error.

When you assign a property that does not exist to a variable using the object destructuring, the variable is set to undefined. For example:

let { firstName, lastName, middleName } = person;
console.log(middleName); // undefinedCode language: JavaScript (javascript)

In this example, the middleName property doesn’t exist in the person object, therefore, the middleName variable is undefined.

Setting default values

You can assign a default value to the variable when the property of an object doesn’t exist. For example:

let person = {
    firstName: 'John',
    lastName: 'Doe',
    currentAge: 28
};

let { firstName, lastName, middleName = '', currentAge: age = 18 } = person;

console.log(middleName); // ''
console.log(age); // 28Code language: JavaScript (javascript)

In this example, we assign an empty string to the middleName variable when the person object doesn’t have the middleName property.

Also, we assign the currentAge property to the age variable with the default value of 18.

However, when the person object does have the middleName property, the assignment works as usual:

let person = {
    firstName: 'John',
    lastName: 'Doe',
    middleName: 'C.',
    currentAge: 28
};

let { firstName,  lastName, middleName = '', currentAge: age = 18 } = person;

console.log(middleName); // 'C.'
console.log(age); // 28Code language: JavaScript (javascript)

Destructuring a null object

A function may return an object or null in some situations. For example:

function getPerson() {
    return null;
}Code language: JavaScript (javascript)

And you use the object destructuring assignment:

let { firstName, lastName } = getPerson();

console.log(firstName, lastName);Code language: JavaScript (javascript)

The code will throw a TypeError:

TypeError: Cannot destructure property 'firstName' of 'getPerson(...)' as it is null.Code language: JavaScript (javascript)

To avoid this, you can use the OR operator (||) to fallback the null object to an empty object:

let { firstName, lastName } = getPerson() || {};Code language: JavaScript (javascript)

Now, no error will occur. And the firstName and lastName will be undefined.

Nested object destructuring

Assuming that you have an employee object which has a name object as the property:

let employee = {
    id: 1001,
    name: {
        firstName: 'John',
        lastName: 'Doe'
    }
};Code language: JavaScript (javascript)

The following statement destructures the properties of the nested name object into individual variables:

let {
    name: {
        firstName,
        lastName
    }
} = employee;

console.log(firstName); // John
console.log(lastName); // DoeCode language: JavaScript (javascript)

It’s possible to do multiple assignment of a property to multiple variables:

let employee = {
    id: 1001,
    name: {
        firstName: 'John',
        lastName: 'Doe'
    }
};

let {
    name: {
        firstName,
        lastName
    },
    name
} = employee;

console.log(firstName); // John
console.log(lastName); // Doe
console.log(name); // { firstName: 'John', lastName: 'Doe' }Code language: JavaScript (javascript)

Destructuring function arguments

Suppose you have a function that displays the person object:

let display = (person) => console.log(`${person.firstName} ${person.lastName}`);

let person = {
    firstName: 'John',
    lastName: 'Doe'
};

display(person);Code language: JavaScript (javascript)

It’s possible to destructure the object argument passed into the function like this:

let display = ({firstName, lastName}) => console.log(`${firstName} ${lastName}`);

let person = {
    firstName: 'John',
    lastName: 'Doe'
};

display(person);Code language: JavaScript (javascript)

It looks less verbose especially when you use many properties of the argument object. This technique is often used in React.

Summary

Object destructuring assigns the properties of an object to variables with the same names by default.

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