What is destructuring assignment and its uses?

What is destructuring assignment ?

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

- MDN

Advantages

A. Makes code concise and more readable.

B. We can easily avoid repeated destructing expression.

Some use cases

1. To get values in variable from Objects,array

let obj = { 'a': 1,'b': {'b1': '1.1'}}
let {a,b,b:{b1}} = obj
console.log('a--> ' + a, '\nb--> ', b, '\nb1---> ', b1)

let obj2 = { foo: 'foo' };
let { foo: newVarName } = obj2;
console.log(newVarName);

let arr = [1, 2, 3, 4, 5]
let [first, second, ...rest] = arr
console.log(first, '\n', second, '\n', rest)


// Nested extraction is possible too:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);

2. To combine an array at any place with another array.

let arr = [2,3,4,5]
let newArr = [0,1,...arr,6,7]
console.log(newArr)

3. To change only desired property in an object

let arr = [{a:1, b:2, c:3},{a:4, b:5, c:6},{a:7, b:8, c:9}]

let op = arr.map( ( {a,...rest}, index) => ({...rest,a:index+10}))

console.log(op)

4. To create a shallow copy of objects

let obj = {a:1,b:2,c:3}
let newObj = {...obj}
newObj.a = 'new Obj a'

console.log('Original Object', obj)
console.log('Shallow copied Object', newObj)

5. To extract values from parameters into standalone variables

// Object destructuring:
const fn = ({ prop }) => {
  console.log(prop);
};
fn({ prop: 'foo' });


console.log('------------------');


// Array destructuring:
const fn2 = ([item1, item2]) => {
  console.log(item1);
  console.log(item2);
};
fn2(['bar', 'baz']);


console.log('------------------');


// Assigning default values to destructured properties:

const fn3 = ({ foo="defaultFooVal", bar }) => {
  console.log(foo, bar);
};
fn3({ bar: 'bar' });

6. To get dynamic keys value from object

let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj

console.log(value)

7. To build an object from other object with some default values

let obj = {a:1,b:2,c:3}
let newObj = (({d=4,...rest} = obj), {d,...rest})
console.log(newObj)

8. To swap values

const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // swap index 0 and 2

console.log(b);

9. To get a subset of an object

• 9.1 subset of an object:

  Show code snippet

  const obj = {a:1, b:2, c:3},
  subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function
  
  console.log(subset);

  Run code snippetHide results

  Expand snippet

• 9.2 To get a subset of an object using comma operator and destructuring:

  Show code snippet

  const object = { a: 5, b: 6, c: 7  };
  const picked = ({a,c}=object, {a,c})
  
  console.log(picked); // { a: 5, c: 7 }

  Run code snippetHide results

  Expand snippet

10. To do array to object conversion:

const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);

console.log(date);

11. To set default values in function. (Read this answer for more info )

function someName(element, input,settings={i:"#1d252c", i2:"#fff",...input}){
    console.log(settings.i)
    console.log(settings.i2)
}

someName('hello', {i:'#123'})
someName('hello', {i2:'#123'})

12. To get properties such as length from an array, function name, number of arguments etc.

let arr = [1,2,3,4,5];

let {length} = arr;

console.log(length);

let func = function dummyFunc(a,b,c) {
  return 'A B and C';
}

let {name, length:funcLen} = func;

console.log(name, funcLen);

It is something like what you have can be extracted with the same variable name

The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment

var [one, two, three] = ['orange', 'mango', 'banana'];

console.log(one); // "orange"
console.log(two); // "mango"
console.log(three); // "banana"

and you can get user properties of an object using destructuring assignment,

var {name, age} = {name: 'John', age: 32};

console.log(name); // John
console.log(age); // 32

The De-structured assignment of Javascript is probably an inspiration drawn from Perl language.

This facilitates reuse by avoid writing getter methods or wrapper functions.

One best example that I found very helpful in particular was on reusing functions that return more data than what is required.

If there is a function that returns a list or an array or a json, and we are interested in only the first item of the list or array or json, then we can simply use the de-structured assignment instead of writing a new wrapper function to extract the interesting data item.