Register to get access to 15+ free programming courses with interactive exercises

Parameter destructuring JS: Functions

When we pass an argument when we call a function, its value is assigned to the function parameter. This is an implicit automatic assignment, because this operation is "not visible" in the code.

const func = (x) => {
  // the x parameter will be assigned to
  // argument value when the function is called
  console.log(x);
};

func(1); // => 1
// It can be imagined that
// the x parameter is created inside the function,
// to which the value of the argument is assigned:
// {
//   let x = 1;
//   console.log(x);
// };

func([1, 2]); // => [1, 2]
// An example of an array transfer:
// {
//   let x = [1, 2];
//   console.log(x);
// };

Passing an argument is no different from simply assigning a value to a variable. This means that we have access to destructuring right in the function definition.

Array destructuring

Let's write a function that takes as input an array of two elements and prints them to the terminal. Let's look at different ways of implementing parameter handling.

Not the most expressive variant - direct reference to array elements by indexes:

const func = (elements) => {
  console.log(elements[0]);
  console.log(elements[1]);
};

// let elements = [1, 2];
func([1, 2]);
// => 1
// => 2

A more interesting option is to destruct the array in the body of the function:

const func = (elements) => {
  const [first, second] = elements;
  console.log(first);
  console.log(second);
};

// let elements = [1, 2];
func([1, 2]);
// => 1
// => 2

https://repl.it/@hexlet/js-functions-params-destructuring-in-function-body

But you can go even further and add destructuring right into the definition:

const func = ([first, second]) => {
  console.log(first);
  console.log(second);
};

// let [first, second] = [1, 2];
func([1, 2]);
// => 1
// => 2

https://repl.it/@hexlet/js-functions-params-destructuring-input-argument

All the standard rules of array destructuring apply:

const elements = [1, 2];

// let [first, second] = elements;
func(elements);
// => 1
// => 2

// let [first, second] = [1];
func([1]);
// => 1
// => undefined

// let [first, second] = [];
func([]);
// => undefined
// => undefined

If there are less than two elements in the passed array, the parameters that "lacked" the corresponding values will contain undefined. For such cases, you can insure and set a default value:

const func = ([first = 666, second = 777]) => {
  console.log(first);
  console.log(second);
};

// [first = 666, second = 777] = [1];
func([1]);
// => 1
// => 777

// [first = 666, second = 777] = [];
func([])
// => 666
// => 777

https://repl.it/@hexlet/js-functions-params-destructuring-default-value

Destructuring of the object

Let's write a function that takes as input an object with information about the name and surname of a user and outputs it to the terminal. Immediately we implement the variant with destructuring of the object for parameters:

const func = ({ name, surname }) => {
  console.log(name);
  console.log(surname);
};

// let { name, surname } = { name: 'John', surname: 'Doe' };
func({ name: 'John', surname: 'Doe' });
// => John
// => Doe

https://repl.it/@hexlet/js-functions-params-destructuring-object

A typical situation in practice is when an object with many properties comes to the input of a function, but in fact we do not need the values of all properties, but only a few. This can happen, for example, when processing an HTTP response from a server or configuration for a program. In such cases, we take away only the values we need - because we don't have to specify all of the object's properties when destructuring:

const func = ({ surname }) => {
  // take only surname values
  console.log(surname);
};

const obj = { name: 'John', surname: 'Doe' };

// let { surname } = { name: 'John', surname: 'Doe' };
func(obj); // => Doe

Recommended materials

  1. Desctructuring lesson from JS: Arrays course
  2. Desctructuring lesson from JS: Objects course

Аватары экспертов Хекслета

Are there any more questions? Ask them in the Discussion section.

The Hexlet support team or other students will answer you.

For full access to the course you need a professional subscription.

A professional subscription will give you full access to all Hexlet courses, projects and lifetime access to the theory of lessons learned. You can cancel your subscription at any time.

Get access
130
courses
1000
exercises
2000+
hours of theory
3200
tests

Sign up

Programming courses for beginners and experienced developers. Start training for free

  • 130 courses, 2000+ hours of theory
  • 1000 practical tasks in a browser
  • 360 000 students
By sending this form, you agree to our Personal Policy and Service Conditions

Our graduates work in companies:

<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.bookmate">Bookmate</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.healthsamurai">Healthsamurai</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.dualboot">Dualboot</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.abbyy">Abbyy</span>
Suggested learning programs

From zero to a developer. Refunds in case you won't get a job

Frontend Developer icon
Profession
New
Development of front-end components for web applications
start anytime 10 months

Use Hexlet to the fullest extent!

  • Ask questions about the lesson
  • Test your knowledge in quizzes
  • Practice in your browser
  • Track your progress

Sign up or sign in

By sending this form, you agree to our Personal Policy and Service Conditions

Toto Image

Ask questions if you want to discuss a theory or an exercise. Hexlet Support Team and experienced community members can help find answers and solve a problem.