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:
const obj = {a:1, b:2, c:3}, subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function console.log(subset); -
9.2 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 }
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);