Declaration of key variables in Javascript

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I recently saw a type of Javascript code, in some examples of Electron, which I believe is part of ES6. These are variable statements in the following format:

const electron = require('electron');
// Module to control application life.
const {app} = electron;
// Module to create native browser window.
const {BrowserWindow} = electron;

Here is being used with const, but I don’t know if this also exists with let and var.

The important thing here is that instead of declaring in the usual way

const app = electron;
const BrowserWindow = electron;

We’re putting app and BrowserWindow between keys.

What does this type of declaration mean with the variable name inside keys? When and how should this type of declaration be used?

javascript ecmascript-6 variable-declaration
  • 1

    Sérgio already answered your question, but if you want to declare these keys to get several properties of an object it would be better to declare the with instead. Example: var obj = {prop: 5, ah: 2, uuh: true, y: false}; with(obj) { alert(prop); }. The with expands an object and is supported from ES1.

1 answer

9


This is a new technology called destructuring assignment and which in practice is a shortcut for declaring variables with the same name as an object property, or in arrays declare indexing to the position of an array.

For example:

var obj = {
  foo: 20,
  bar: 50
};

const {foo} = obj;

console.log(foo);

is a shortcut of 

const foo = obj.foo;

This is practical and can be used in more than one at a time:

const {foo, bar} = obj;

which is even that 

const foo = obj.foo;
const bar = obj.bar;

This can also be done in arrays, in this case the array positions are imported:

var arr = [0, 5, 10];
const [primeiro, segundo] = arr;

console.log(primeiro, segundo); // 0, 5

which is the same as:

var primeiro = arr[0];
var segundo = arr[1];

Another very useful way is in function arguments:

var Ana = {
    nome: 'Ana',
    idade: 34
}

function info({nome,idade}) {
    console.log(`Pessoa: ${nome}, Idade: ${idade}`);
}

info(Ana); // Pessoa: Ana, Idade: 34

Notes:

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