CODEWITHSUNDEEP
javascriptnode.jssyntaxparametersarguments
Jay Jeong
Jay Jeong

Reputation: 994

I don't quite get a javascript syntax about passing arguments

My code:

const users = [ { id: 1, name: 'user1' }, { id: 2, name: 'user2' } ]

p = new Promise( resolve => resolve(users) )

p.then( (user) => console.log(user) )

Return me the following logs:

[ { id: 1, name: 'user1' }, { id: 2, name: 'user2' } ]

If I change it as below,

users.then( ([user]) => console.log(user) )

I receive the following logs:

{ id: 1, name: 'user1' }

I don't quite understand why the second one only logs the first element in the array.

Upvotes: 2

Views: 50

Answers (2)

Orelsanpls
Orelsanpls

Reputation: 23495

Destructuring is an ES6 aka ECMA2015 features.

You can destructure Array and Objects.

Object destructuring as bellow, allow you to pick up the keys you want to get from an object using easy syntax.

const obj = {
  a: 'a',
  b: 'b',
  c: 'c',
};

// Without object destructuring
const a_ = obj.a;
const b_ = obj.b;


// With object destructuring
const {
  a,
  b,
} = obj;

console.log(a);

Array destructuring is quite the same, but instead of specifying the keys you want to get, you use the index of the data.

const arr = [
  'a',
  'b',
  'c',
];

// Without array destructuring
const a_ = arr[0];
const c_ = arr[2];

// With array destructuring
const [
  a, , c,
] = arr;

console.log(c);

You can use Array and Object destructuring anywhere; including in function parameters. It's especially good in this case because you can assign default value easily to keys, as bellow

function func({
  a,
  b,
  c = 'default',
}) {
  console.log(a, '/', b, '/', c);
}

func({
  a: 'j\'aime la france',
  b: 'vive les baguettes',
});

Upvotes: 0

Francesco
Francesco

Reputation: 4250

It's a destructuring assignment

[a, b, c] = [1, 2, 3]

console.log(a)
console.log(b)
console.log(c)

but if you only destructure one, you get the first value of the array

[a] = [1, 2, 3]

console.log(a)

As for your example, destructuring can be done in many places such as function arguments

Upvotes: 4

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