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I have been reading about 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?
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);
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.
I have been reading about 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?
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);
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.
I have been reading about 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?
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);
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.
I have two JSON arrays like
array1=[{a:1,b:2,c:3,d:4}]
&
array2=[{a:2,b:5,c:3,d:4}]
Is there any method to find the value of one of the keys in array1 present in array 2.Here in the array 1 key b contains the value 2,and array2 also contain a key a with value 2. How can I capture the key name of array 2 which has the same value for one of the keys in array.
I don't quite understand if you are interested in operating on arrays or objects - as your example is a pair of single element arrays, and the comparison is clearly between the objects in the arrays.
That said, if your goal is to compare two objects, and return the set of keys that are the same for both, you would do something like
obj1 = {a:1,b:2,c:3,d:4};
obj2 = {a:2,b:5,c:3,d:4};
function sameKeys(a,b) {
return Object.keys(a).filter(function(key) {
return a[key] === b[key];
});
}
console.log(sameKeys(obj1, obj2));
When I run this, I get:
[ 'c', 'd' ]
I hope that is what you were asking...
Wrote a prototype function to compare an object against another.
var obj1 = {a: 1, b: 2, c: 3, d: 4};
var obj2 = {a: 2, b: 4, c: 100, d: 200}
Object.prototype.propertiesOf = function(visitorObj) {
result = {};
//Go through the host object
for (thisKey in this) {
//Exclude this function
if (!this.hasOwnProperty(thisKey))
continue;
//Compare the visitor object against the current property
for (visitorKey in visitorObj) {
if (visitorObj[visitorKey] === this[thisKey])
result[visitorKey] = thisKey;
}
}
return result;
}
console.log(obj1.propertiesOf(obj2));
Simply call the propertiesOf function of any object by passing another object as the argument. It returns an object which has similar keys linked to each other.
The above example will result in:
{a: "b", b: "d"}
It seems you want something like this: make a function that finds all the keys in the 2nd object that have a given value. Then pass the value from the first object to that function.
obj1={a:1,b:2,c:3,d:4};
obj2={a:2,b:5,c:3,d:4};
function findKeysByValue(obj, v) {
var results = [];
for (var k in obj) {
if (obj.hasOwnProperty(k) && v == obj[k]) {
results.push(k);
}
}
return results;
}
console.log(findKeysByValue(obj2, obj1['b'])); // ['a']
I use a Javascript hash object to store a set of numerical counters, set up like this [this is greatly simplified]:
var myHash = {
A: 0,
B: 0,
C: 0
};
Is there a way to make other keys, ones not explicitly in myHash, map to keys that are? For instance, I'd like [again, this is simplified]:
myHash['A_prime']++; // or myHash.A_prime++;
to be exactly equivalent to
myHash['A']++; // or myHash.A++;
e.g. incrementing the value found at the key A, not A_prime.
Assuming there aren't any valid string values in this hash, you could create your own mapping of keys to other keys, within the hash itself, and wrap the key/value pairs in your own function, as follows:
var hash = {
A: 0,
B: 0,
C: 0,
A_asdf: 'A',
B_asdf: 'B',
A_another: 'A'
}
function increment(key)
{
// iteratively find the real key
while (typeof(hash[key]) == 'string')
{
key = hash[key];
}
hash[key]++;
}
A way of solving this would be wrapping all values in an object, and use this object in several keys.
var myHash = {
A: {value: 0},
...
};
myHash['A_prime'] = myHash['A'];
myHash['A_prime'].value++;
I think you would be better off just writing a function to accomplish the incrementation of one hash array or multiple depending on what you are trying to accomplish. I may have not understood the question correctly but this is my solution.
var smallHash = {
A: 0,
B: 0,
C: 0,
A_prime:'A',
B_prime:'B',
C_prime:'C'
};
function incrementPrime(myHash,key)
{
letterKey = myHash[key];
myHash[letterKey]++;
}
incrementPrime(smallHash,'C_prime');
alert("C is now: "+smallHash['C']); // should return 1
You could act on the key, instead of the hash:
var myHash = {
A: 0,
B: 0,
C: 0
};
function incr(prop){
myHash[prop[0]]++;
}
incr('A');
incr('A_asdf');
The function incr could be anything else you want to apply to the hash.
As described, no. If you can change your keys to objects though you'd get closer:
var x = new Object();
var y = x;
myHash[x] = 1;
myHash[y]++; // should increment the value since y is x.