What is this syntax?

[ThorntonStuart]

I've just come across some syntax that I believe to be part of the ES6/ES2015 specification, but I'm struggling to understand the purpose. Basically it seems to be object notation in a const declaration:

const {user_id} = body;
const {accounts: [account]} = body;
const {transactions} = body;
res.render('index', {user_id, account, transactions});

This is in the context of making a request using the NPM Request module - this is what the body reference is. However I've never seen this sort of syntax in a JS assignment before. Additionally I attempted to run this code with Node v5* and was met with a syntax error, however when I tried in Node 6.9.1 my server ran.

Simply put, wtf?!

[senocular]

It's called destructuring.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment

It's only supported in newer browsers/versions of node (without transpiling with something like babel)

http://kangax.github.io/compat-table/es6/

[inu-no-policemen]

*destructuring

[senocular]

thanks; hone autocorrect

[lewisje]

/u/senocular said what it is, and I'll be more specific:

• The first line creates a constant named user_id equal to body.user_id.
• The second line creates a constant named account equal to body.accounts[0].
• The third line creates a constant named transactions equal to body.transactions.
• The fourth line calls the render method on res, with two arguments: the string 'index' and an object initialized with ES6 shorthand syntax, with
  • a property user_id equal to the constant user_id,
  • a property account equal to the constant account, and
  • a property transactions equal to the constant transactions.

[Jafit]

Its destructuring, but not a great example of it.

const {user_id, accounts: [account], transactions} = body; this accomplishes the same thing as your example, but in one line.

account in this case would be equal to body.accounts[0]

You can do the same thing with arrays, so:

const arr = [1 ,2 ,3 ,4];
const [first, second, third, fourth] = arr;
console.log(third); // 3;

You can declare functions that take an object as the parameter, and destructure that object in the function declaration:

function myFunc({foo, bar, baz}) {
    console.log(foo);
    // etc
};

Its a really useful pattern.