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Destructuring assignment in ES6 for more clarity

ES6 introduces the Destructuring assignment that allows javascript developers to assign items of arrays or properties of objects to any individual variables. While the syntax is more simplified it exhibits more clarity that the traditional property access. 

Here is how you would extract items from an array in ES5:

var animals = ['dog','lion','cat']
var a = animals[0];
var b = animals[1];
var c = animals[2];
console.log(a); //dog
console.log(b); //lion
console.log(c); //cat

In ES6, this is more easy and simplified: 

var[a,b,c] = ['dog','lion','cat'];
console.log(a); //dog
console.log(b); //lion
console.log(c); //cat

Destructuring works with arrays and objects. Essentially you can assign anything iterable on the right-side to anything at the left-side.

Array Destructuring

Here we’ll see how we can utilize the Destructuring Assignment for an array.

The assignment is separate from the declaration

We can assign the value to the variable by destructuring the variable from its declaration.

var a, b;
[a, b] = [1, 2];
console.log(a); // 1
console.log(b); // 2

Default values

If you are not sure that the array may contain all required values, provide default variables while destructuring.

var [a=5, b=7] = [1];
console.log(a); // 1
console.log(b); //

Swapping variables

Traditionally, swapping two values requires a temporary variable. However, a single destructuring expression does this without the need for an extra variable.

var a = 1;
var b = 3;
[a, b] = [b, a];
console.log(a); // 3
console.log(b); // 1

Ignoring values

Ignoring a value from an array is as simple as inserting white space in place of the variable for that specific value at the right-hand side.

var [a, , b] = [1, 2, 3];
console.log(a); // 1
console.log(b); // 3

Rest in array destructuring

While destructuring an array, you can assign the first few items to individual variables. For the rest, you can create a new array.

var [a, b, …rest] = [1, 2, 3, 4, 5, 6];
console.log(a); // 1
console.log(b); // 2
console.log(rest); // [3,4,5,6]

The syntax is a spread operator. More detail about spread operators is available here.

Unpackaging an array passed to a function parameter

Destructuring can unpack array values to use them inside a function where an array is passed as a parameter.

var testArray = [1,2,6,3];
testingArray(testArray);
function testingArray([a,b]){
  console.log(a);
  console.log(b);
}

Object Destructuring

The Destructuring assignment is also used for objects. We can easily unpack object properties and assign them to new variables. Let’s see the implementation of a destructuring assignment for objects.

Basic Assignment

While destructuring an object, the variable names should be the same as the object properties. Here is an example of a basic assignment expression for objects:

var o = {p: 42, q: true};
var {p, q} = o;
console.log(p); // 42
console.log(q); // true

An assignment without a declaration

We can destructure object properties and assign them to the variable, while separately declaring the variable.

var a, b;
({a, b} = {a: 1, b: 2});
console.log(a); // 1
console.log(b); // 2

While using object literals for a destructuring assignment without a declaration, parentheses (…) are required around the assignment statement. Brackets {..} on the left-hand side considered as a block and not an object literal.

{a, b} = {a: 1, b: 2}; // this would be an invalid statement without parentheses.

Assigning new variable names

We can assign a different name than the property to a variable while destructuring an object.

var o = {p: 42, q: true};
var {p: foo, q: bar} = o;

console.log(foo); // 42
console.log(bar); // true

Default values

While destructuring an object we can provide the default values for undefined properties of that particular object.

var {a = 10, b = 5} = {a: 3};
console.log(a); // 3
console.log(b); // 5

Rest in object destructuring

Same as an array we can assign the first few properties of the object to variables, and with the rest of the properties, we can create a new object.

let {a, b, …rest} = {a: 10, b: 20, c: 30, d: 40}
console.log(a); // 10
console.log(b); // 20
console.log(rest); // { c: 30, d: 40 }

Unpackaging an object passed to a function parameter

When an object is passed to the function as a parameter, we can destructure the required object properties to use them inside the function.

function studentFullName({fullName: {firstName, lastName}}){
  return `${firstName} ${lastName}`;
}

var student = {
  id: 42,
  displayName: ‘Robin’,
  fullName: {
    firstName: ‘Robin’,
    lastName: ‘Wine’
  }
};

console.log(studentFullName(student));// Robin Wine

Conclusion

Destructuring assignments make it easy to extract the required values or properties from any iterable. Destructuring assignment destructurizes the array or object by copying the items into variables, without modifying the array or object itself and that’s the beauty of it.

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About the Writer

  • jayesh raipure
    Jayesh Raipure

    Jayesh is a senior software developer at Synerzip. He has spent more than four years in PHP and web development. Jayesh has extensive experience in Laravel, WordPress, Angular and AWS. He holds a Bachelors in Computer Science degree from Rajiv Gandhi Proudyogiki Vishwavidyalaya, Bhopal. 

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