What is destructuring assignment and its uses?

Question

I have been reading about [Destructuring assignment](https://developer.mozilla.org/en- US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment) introduced in ES6.

What is the purpose of this syntax, why was it introduced, and what are some examples of how it might be used in practice?

Answer

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.

- [From MDN](https://developer.mozilla.org/en- US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment)

Advantages

Makes code concise and more readable.
We can easily avoid repeated destructing expression.

Some use cases

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);

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)

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' });

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)

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);

To get a subset of an object

subset of an object:

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

console.log(subset);

To get a subset of an object using comma operator and destructuring:
```
      const object = { a: 5, b: 6, c: 7  };
```
const picked = ({a,c}=object, {a,c})

console.log(picked); // { a: 5, c: 7 }

To do array to object conversion:

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

console.log(date);

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'});

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);