In my opinion, and consdering your needs, get/set is a better approach, they are faster in execution time, and the code is easier to mantain.

GETTERS/SETTERS SOLUTION

Are arround since the arrival of ES5...

• A getter is a method that gets the value of a specific property.
• A setter is a method that sets the value of a specific property.

You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties. The syntax for defining getters and setters uses the object literal syntax.

+info : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Working_with_Objects

var box = { 
    _number : 0,
    get number(){ return this._number },
    set number(value){ 
        /* your verifications here */
        if( typeof value !== 'number' ) throw new Error('Invalid input')
        /* if validates , asign value */
        this._number = value;
    }
}
// test...
box.number = "1234"; // FAIL
box.number = 1234;   // OK
box.number;          // output = 1234

PROXY SOLUTION

Available since ES6. Probably not appropiate if performance is important for you. Use the GET/SET proxy traps, to obtain the same behavior as in the previous example.

// your original object...
var box = {};
// create a proxy linked to your original object
var boxProxy = new Proxy( box , {
    get( obj, prop, receiver ){ return obj[prop] },
    set( obj, prop, value){ 
        /* check if the property trying to be written is 'number' */
        if( prop === 'number' ){
             /* your verifications here */
            if( typeof value  !== 'number' ) throw new Error('Invalid input')
            /* if validates , asign value */
            this._number = value;
        }
    } 
 });
// test...
boxProxy.number = "1234"; // FAIL
boxProxy.number = 1234;   // OK
boxProxy.number;          // output = 1234

In both cases, if you require the propery box._number to be private and hidden you may implement it, using closures.

Here is an example, you have a verification code, and a default value

var box =Object.create({},{
number: {    
    get: function() { return this._number || 0; },
    set: function(value) {
      if(typeof value!='number') throw new Error('error')
      this._number=value
    }
}
})

If you want to hide the private _number you can wrap in a function

var box=(function() {
    var _number
    var box =Object.create({},{
    number: {    
        get: function() { return _number || 0; },
        set: function(value) {
          if(typeof value!='number') throw new Error('error')
          _number=value
        }
    }
    })
    return box
})()

box.number=3

More info:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create