What is destructuring assignment and its uses?

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深忆病人
深忆病人 2020-11-22 00:28

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

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  • 2020-11-22 01:19

    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.

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  • 2020-11-22 01:21

    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)

    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);

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  • 2020-11-22 01:22

    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
    
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