How to scope a variable to a class in javascript

Question

I am trying to create a class using the class foo {} syntax, and want a private variable inside of it. Right now, I have something like this:

'use strict';
class foo {
  constructor(x1,y1) {
    var x = x1;
    var y = y1;
  }
  get x() {
    return x;
  }
  get y() {
    return y;
  }
}

Whenever I try to access foo.x and foo.y, I get a x is not found error. However, when I tried putting the variable declarations outside of the constructor, it stopped working. I'm also using static methods, so I can't abandon the class syntax without some work. How can I fix this? How can I declare a variable inside of the object, that is global and can't be found outside?

@GabrieleB-David declaring this.x would allow access to x from outside the class. — pizzarob
– pizzarob, Commented Mar 23, 2017 at 20:49
@realseanp you're right. I missed the part where he said "private variable". I guess my elementary school teachers were right all those years ago: read the whole question before answering :P — Gab
– Gab, Commented Mar 23, 2017 at 20:52

Bergi · Accepted Answer · 2017-03-23 20:49:53Z

4

Those getters are declared on the prototype, where they cannot access the private (scoped) variables in the constructor. You'll need to use Object.defineProperty for creating instance-specific getters:

class Foo {
  constructor(x, y) {
    Object.defineProperties(this, {
      x: {get() { return x; }, configurable: true},
      y: {get() { return y; }, configurable: true}
    });
  }
}

This is no different than in ES5. Btw, given you don't assign to the variables, you might as well make them non-writable properties.

answered Mar 23, 2017 at 20:49

Bergi

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2 Comments

Wyrme Over a year ago

How could I make a variable/property that is only writable inside of the instance?

Bergi Over a year ago

@Wyrme x and y (the parameters) already are exactly such variables

Nelson Teixeira · Accepted Answer · 2017-03-23 21:06:36Z

1

Well people are assuming you want to use getters and setters. If that's not the case, this will suffice:

'use strict';
class foo {
  constructor(x1,y1) {
    this.x = x1;
    this.y = y1;
  }
}

var f = new foo(10, 20);

console.log(f.x)
console.log(f.y)

JSFiddle: https://jsfiddle.net/gLxnqfrv/

answered Mar 23, 2017 at 21:06

Nelson Teixeira

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Comments

Cookie · Accepted Answer · 2017-03-23 21:03:41Z

0

Taken from es6-features this is the correct syntax for a class and it's getters and setters.

class Rectangle {
    constructor (width, height) {
        this._width  = width
        this._height = height
    }
    set width  (width)  { this._width = width               }
    get width  ()       { return this._width                }
    set height (height) { this._height = height             }
    get height ()       { return this._height               }
    get area   ()       { return this._width * this._height }
}
var r = new Rectangle(50, 20)
r.area === 1000

Important to note that there is no requirement to damage the prototype in ES6.

edited Mar 23, 2017 at 21:03

answered Mar 23, 2017 at 20:55

Cookie

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