How to destructure into dynamically named variables in ES6?

74
votes

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

5
javascriptecmascript-6
Here's a related question about destructuring all properties: stackoverflow.com/questions/31907970/… - Probably the same answer applies here – CodingIntrigue
If fields were changed to be an empty array then you would be creating no variables and any code after that would be jeopardized. Using a const with literals ensures that risk could be determined beforehand but something like fields = nonliteralvar would create problems. – Michael Theriot

5 Answers

14
votes

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.

55
votes

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:

9
votes

As discussed before, you can't destruct into dynamically named variables in JavaScript without using eval.

But you can get a subset of the object dynamically, using reduce function as follows:

const destruct = (obj, ...keys) => 
  keys.reduce((a, c) => ({ ...a, [c]: obj[c] }), {});

const object = { 
  color: 'red',
  size: 'big',
  amount: 10,
};

const subset1 = destruct(object, 'color');
const subset2 = destruct(object, 'color', 'amount', 'size');
console.log(subset1);
console.log(subset2);
7
votes

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);
4
votes

You can't destruct without knowing the name of the keys or using an alias for named variables

// you know the name of the keys
const { id, fullName } = user;

// use an alias for named variables
const { [fields[0]]: id, [fields[1]]: fullName } = user;

A solution is to use Array.reduce() to create an object with the dynamic keys like this:

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

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

const obj = fields.reduce((acc, k) => ({ ...acc, ...(user.hasOwnProperty(k) && { [k]: user[k] }) }), {});

for(let k in obj) {
  console.log(k, obj[k]);
}