How to bind methods when destructuring an object in JavaScript?

Question

How to bind methods when destructuring an object in JavaScript?

const person = {
  getName: function() {
    console.log(this);
  }
};

var a = person.getName;
var b = person.getName.bind(person);
var {getName: c} = person;

person.getName(); //=> {getName: [Function]}
a();              //=> window or global
b();              //=> {getName: [Function]}
c();              //=> window or global

I want c to log in the console its "parent" object {getName: [Function]}.

Is there any way to bind all methods when destructuring an object in one destructuring line?

Answer 1

No, there is no way. Functions detached from objects lose the original context. And destructing in JavaScript has no syntax to do something with extracted values on the fly.

Answer 2

There is a simple workaround using ES6 classes. You can use bind in the class constructor to manually set the context of the function.

In the example below, getName() will "survive" the destructuring :

class Person {
  constructor() {
    this.getName = this.getName.bind(this);
  }

  getName() {
    console.log(this);
  }
}

const {
  getName
} = new Person();

getName(); // Person { getName: [Function: bound getName] }

Answer 3

Just use an arrow method:

const person = {
  getName: () => console.log(this),
};

Answer 4

You can use a getter or a proxy to bind a method whenever you get it, even using destructuring.

Both solutions check if method is already bound, by looking for bound at the start of the name using String.startsWith(). If not bound, that method will be bound before returning it.

1. Auto-bind the method to the object with a getter. This will require a getter for each method.

const person = {
  prop: 5,
  _getName: function() {
    console.log(this.prop);
  },
  
  get getName() {
    // if not bound, bind the method
    if(!this._getName.name.startsWith('bound ')) {
      this._getName = this._getName.bind(this);
    }
    
    return this._getName;
  }
};

var a = person.getName;
var b = person.getName.bind(person);
var {getName: c} = person;

person.getName(); //=> 5
a();              //=> 5
b();              //=> 5
c();              //=> 5

2. Auto-bind the method to the object with a proxy. Define once for all methods.

var handler = {
  get: function(target, prop, receiver) {
    // if method, and not bound, bind the method
    if(typeof target[prop] === 'function' && !target[prop].name.startsWith('bound ')) {
      target[prop] = target[prop].bind(target);
    }
    
    return target[prop];
  }
};

const person = new Proxy({
  prop: 5,
  getName: function() {
    console.log(this.prop);
  }
}, handler);

var a = person.getName;
var b = person.getName.bind(person);
var {getName: c} = person;

person.getName(); //=> 5
a();              //=> 5
b();              //=> 5
c();              //=> 5

Answer 5

Yes. We can do this with a single line of code, assuming I understood your question correctly.

This certainly isn't the most readable approach, but it's a method that I use often in my personal projects:

const person = {
    getName: function() {
      console.log(this);
    }
  };

// IIFE for destructuring and applying changes inline
const { a, b, c } = (({ getName }) => ({ a: getName.bind(person), b: getName.bind(person), c: getName.bind(person) }))(person);

Now we call the destructured props:

a(); // { getName: [Function: getName] }
b(); // { getName: [Function: getName] }
c(); // { getName: [Function: getName] }

What's happening here? If that line was confusing, read on...

We are wrapping everything in an IIFE which accepts as input the person object; we pull the getName prop inline.¹

Finally, we are explicitly mapping each call to bind using the getName prop - as pulled from our sole argument, person - to correspond to a, b, c, which we are destructuring away from the resolved IIFE.

¹OK so I've arguably cheated here in that we have two destructuring "statements" but which are ultimately resolved in the same expression