JavaScript - 配列とオブジェクトの構造化 [ライブドキュメント]

• 新しいトピックを個別に勉強してください。そうしないと、長期にわたって概念を完全に理解できなくなります。これはいくつかの実証研究によっても裏付けられています。
• 分解: 配列またはオブジェクトの値を個別の変数に解凍する方法。

const nums = [8,4,5];
const num1 = nums[0];
const num2 = nums[1];
const num3 = nums[2];
console.log(num1, num2, num3);

is reduced to 

const [x,y,z] = nums;
console.log(x, y, z);

three const variables named x,y,z are created in this step

• [x,y,z] は配列のように見えますが、= の左側にある場合、構造化されているとみなされます。
• 破壊は不変操作です。

const girl = {
  name: 'Melania',
  friends: ['Alina', 'Alice', 'Ayesha', 'Anamika', 'Anaya'],
  eats: ['Roasted', 'Pizza', 'Burger', 'Rice', 'Manchurian'],
};

const [first, second] = girl.friends;
console.log(first, second);

const [,,,fourth,last] = girl.eats;
console.log(fourth, last);

変数の交換 [変更]

let array = [5,6];
let [a,b] = array;
console.log(`a: ${a}, b:${b}`);
[b,a] = [a,b];
console.log(`a: ${a}, b:${b}`);

関数は配列を返し、結果をすぐに破棄することで、関数から複数の値を返すことができます

const girl = {
  name: 'Melania',
  friends: ['Alina', 'Alice', 'Ayesha', 'Anamika', 'Anaya'],
  eats: ['Roasted', 'Pizza', 'Burger', 'Rice', 'Manchurian'],
  drinks: ['Juice','Coffee','Coke'],
  order: function(eat,drink){
    return [this.eats[eat],this.drinks[drink]];
  }
};

const [mainCourse, drinks] = girl.order(2, 2);

console.log(`mainCOurse: ${mainCourse}`);
console.log(`drinks: ${drinks}`);

入れ子になった配列の構造を解除する

let nums = [5,3,[8,7,9,3]];
let [x,y,z] = nums;
console.log(`x: ${x}`); // 5
console.log(`y: ${y}`); // 3
console.log(`z: ${z}`); // 8,7,9,3

let nums2 = [5,3,[8,7]];
let [x,,[y,z]] = nums2;
console.log(`x: ${x}`, `y: ${y}`, `z: ${z}`); // 5 8 7 

サイズが不明な配列からの構造解除:

const names = ['Michael','Charlie','Peter'];
let [w='XXX',x='XXX',y='XXX',z='XXX'] = names;
console.log(w,x,y,z); // 'Michael' 'Charlie' 'Peter' 'XXX'

オブジェクトの構造化:

• オブジェクトの構造化には {} を使用し、配列の構造化には [] を使用します。
• 抽出するオブジェクトのプロパティ名に記載されている正確な変数名を指定します。これらの変数名の順序は関係ありません。

const girl = {
  name: 'Melania',
  friends: ['Alina', 'Alice', 'Ayesha', 'Anamika', 'Anaya'],
  eats: ['Roasted', 'Pizza', 'Burger', 'Rice', 'Manchurian'],
  drinks: ['Juice','Coffee','Coke'],
  works: {
        mtwt: {
          start: 9,
          end: 5
        },
        fri: {
          start:9,
          end: 3
        }
  }
};

const {name, works, drinks} = girl;
console.log(name);
console.log(works);
console.log(drinks);


// Replace long property names with custom names:
const {name:user, works:timings, drinks:enjoys} = girl;
console.log(user);
console.log(timings);
console.log(enjoys);


//Destructuring data from API calls returned in the form of objects i.e Attaching a default value to a property that does not exist on object received from an API call

// details does not exist, so default value is assigned
const { details = [], eats: loves = [] } = girl;
console.log(details);
// eats exist but is renamed as loves, hence default value won't apply
console.log(loves);

## Mutating Variables using object destructuring
let x = 10;
let y = 20;
let obj = {x:1, y:2, z:3};

{x,y} = obj; // Error
When we start a line with a '{', then JS expects a code-block. And we cannot assign anything to a code-block on LHS using = operator. Hence, an Error is thrown. The error is resolved by wrapping into () as shown below
({x,y} = obj); //{ x: 1, y: 2, z: 3 }

ネストされたオブジェクトの構造を解除する

const girl = {
  name: 'Melania',
  friends: ['Alina', 'Alice', 'Ayesha', 'Anamika', 'Anaya'],
  eats: ['Roasted', 'Pizza', 'Burger', 'Rice', 'Manchurian'],
  drinks: ['Juice','Coffee','Coke'],
  works: {
        mtwt: {
          start: 9,
          end: 5
        },
        fri: {
          start:10,
          end: 2
        }
  }
};

let { fri } = works;
console.log(fri);

// Destructuring the fri object using the same property names start, end
let {fri: {start, end}} = works;
console.log(start, end);

// Further renaming for shortening start as 'b' and end as 'e'
let {fri: {start: b, end: e}} = works;
console.log(b, e);





const girl = {
  name: 'Melania',
  friends: ['Alina', 'Alice', 'Ayesha', 'Anamika', 'Anaya'],
  eats: ['Roasted', 'Pizza', 'Burger', 'Rice', 'Manchurian'],
  drinks: ['Juice','Coffee','Coke'],
  works: {
        mtwt: {
          start: 9,
          end: 5
        },
        fri: {
          start:10,
          end: 2
        }
  },
  // these destructured property-names have to be same as they are passed inside the girl.sleep(). Order need not be same.
  sleep: function ({time='NA', address='NA', color = 'NA', duration='NA'}){
    console.log(`${this.name} sleeps at ${address} for ${duration} in ${color}light for ${duration}. She loves to eat ${this.eats[0]}`);
  }
};

// A single object is passed, which will be destructured by the method inside the object extracting all values via destructuring
girl.sleep({time: '10pm', address:'home', color: 'blue', duration: '7hrs'});

girl.sleep({time: '9pm', duration: '7hrs'});

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