How to destructure into dynamically named variables in ES6?

Question

Let's suppose I have the following object:

const user = { 
  id: 42, 
  displayName: "jdoe",
  fullName: { 
      firstName: "John",
      lastName: "Doe"
  }
};

And that I want only the id and fullName.

I will do the following :

const { id, fullName } = user

Easy-peasy, right?

Now let's suppose that I want to do the destructuring based on the value of another variable called fields.

const fields = [ 'id', 'fullName' ]

Now my question is : How can I do destructuring based on an array of keys?

I shamelessly tried the following without success:

let {[{...fields}]} = user and let {[...fields]} = user. Is there any way that this could be done?

Thank you

Answer 1

It's not impossible to destructure with a dynamic key. To prevent the problem of creating dynamic variables (as Ginden mentioned) you need to provide aliases.

const user = { 
  id: 42, 
  displayName: "jdoe",
  fullName: { 
      firstName: "John",
      lastName: "Doe"
  }
};

const fields = [ 'id', 'fullName' ];
const object = {};

const {[fields[0]]: id, [fields[1]]: fullName} = user;

console.log(id); // 42
console.log(fullName); // { firstName: "John", lastName: "Doe" }

To get around the problem of having to define static aliases for dynamic values, you can assign to an object's dynamic properties. In this simple example, this is the same as reverting the whole destructuring, though :)

const user = { 
  id: 42, 
  displayName: "jdoe",
  fullName: { 
      firstName: "John",
      lastName: "Doe"
  }
};

const fields = [ 'id', 'fullName' ];
const object = {};

({[fields[0]]: object[fields[0]], [fields[1]]: object[fields[1]]} = user);

console.log(object.id); // 42
console.log(object.fullName); // { firstName: "John", lastName: "Doe" }

sources:

• https://twitter.com/ydkjs/status/699845396084846592

• https://github.com/getify/You-Dont-Know-JS/blob/master/es6%20%26%20beyond/ch2.md#not-just-declarations

Answer 2

Short answer: it's impossible and it won't be possible.

Reasoning behind this: it would introduce new dynamically named variables into block scope, effectively being dynamic eval, thus disabling any performance optimization. Dynamic eval that can modify scope in fly was always regarded as extremely dangerous and was removed from ES5 strict mode.

Moreover, it would be a code smell - referencing undefined variables throws ReferenceError, so you would need more boilerplate code to safely handle such dynamic scope.

Answer 3

Paul Kögel's answer is great, but I wanted to give a simpler example for when you need only the value of a dynamic field but don't need to assign it to a dynamic key.

let obj = {x: 3, y: 6};
let dynamicField = 'x';

let {[dynamicField]: value} = obj;

console.log(value);