Javascript destructuring: copy class variables by reference to allow assigning and modifying values

Question

class MyClass {
  constructor() {
    this.points = [];
    // I need to call this after the components are mounted
    // So keeping the setup separate
    this.setupMyClass();
  }

  setupMyClass() {
    let {points} = this;
    points = [...points, {x: 20, y:20}];

    // ugly code 
    // need to repeat 'this.' everytime I use the variable
    // this.points = [...this.points, {x: 20, y: 20}];

    console.log('points', points);
    console.log('this.points', this.points);
  }
}

myClassInstance = new MyClass();

JSFiddle here

Output:

points: [{..}]
this.points: []

I thought arrays were sent by reference while other values are copied by value. This answer supports the same. What's happening here?

I need to access variables of MyClass in a neat way, how do I do it?

@javrd now I need to keep track of what variables I changed and what I didn't. Or assign back all local vars to class variables at the end? — Saravanabalagi Ramachandran, Oct 20 '18 at 10:51
@SaravanabalagiRamachandran No, it doesn't. The push thing is much better here, I misunderstood the point x) — javrd, Oct 20 '18 at 10:52

Ayush Gupta · Accepted Answer · 2018-10-20T10:50:54.443

3

This is because

[...points, {x: 20, y:20}];

created a new array.

let {points} = this;

originally points to the points array belonging to the class instance, but

points = [...points, {x: 20, y:20}];

changes the reference.

You can use .push to maintain the reference as:

points.push({x: 20, y:20});

EDIT to explain more verbose-ally:

[...points, {x: 20, y:20}] creates a new array, so assigning the new array to points does not change the data the points variable(think pointer) points to, but rather changes the pointer itself to a new memory location.

edited Oct 20 '18 at 10:50

answered Oct 20 '18 at 10:41

Ayush Gupta

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1

As I already explained, `[...points, {x: 20, y:20}]` created a new array, so assigning the new array to `points` does not change the data the `points` variable(think pointer) _points_ to, but rather changes the pointer itself to a new memory location. – Ayush Gupta Oct 20 '18 at 10:48
so it is indeed assigning by reference! just understood the point made: i'm throwing away _that_ reference by assigning a new one – Saravanabalagi Ramachandran Oct 20 '18 at 10:50

Ivar · Answer 2 · 2018-10-20T10:56:00.790

When using let {points} = this;, the points variable contains a value and that value is the reference to your array. So when you edit a property of that array as is done in the answer you linked, you modify the referenced array.

But in your case you use points = [...points, {x: 20, y:20}];. Here you assign a new value (a reference to your newly created array) to points, and so the reference to the old array is gone.

In this case you can simply use this.points = [...points, {x: 20, y:20}]; in order to assign it to the this, or push your object directly to this.points using this.points.push({x: 20, y:20}). (For the latter, you don't need to use a destructuring assignment in the first place.)

Javascript destructuring: copy class variables by reference to allow assigning and modifying values

2 Answers2