Dynamic property values instead of function? [duplicate]

Question

Is it possible to give an object a dynamic property, or rather give an object property a dynamic value? For example:

var obj = {
    a: 0,
    b: 5,
    c: //should always be a+b
}

Obviously I could use a method c=function(){return a+b}, but then I would always need to call c as a method using braces (obj.c() instead of obj.c). This wouldn't really be a problem, but I belive it has to work somehow, since the build in length property also is a property and not a function. This property changes dynamically with the object and isn't a function...

var str = "hello world";
console.log(str.length); //11
str = "hello stack overflow";
console.log(str.length); //20

The property value changes without it beeing updated...

I hope you can help me :)

You're looking for ES5 getters. But .length is not dynamic; strings are immutable. — SLaks
– SLaks, Commented Jan 9, 2017 at 16:30
As @SLaks mentions, it is important to note that String#length is not a dynamic property because a string's internal value cannot be changed. — Phylogenesis
– Phylogenesis, Commented Jan 9, 2017 at 16:34
Your string example doesn't prove your point. You are creating a second new string with a different length. The equivalent with your object would be: var obj = {c: 21}; console.log(obj.c); obj = {c: 42}; console.log(obj.c);. See, c also changed "dynamically" ;) — Felix Kling
– Felix Kling, Commented Jan 9, 2017 at 16:44

Ori Drori · Accepted Answer · 2017-01-09 16:45:04Z

4

Define a getter when creating the object, or add it later using Object#defineProperty:

var obj = {
  a: 0,
  b: 5,
  get c() { // assign it at object creation
    return this.a + this.b;
  }
};

console.log(obj.c);

// add it to existing object
Object.defineProperty(obj, 'd', {
  get: function() {
    return this.a + this.b + this.c;
  }
});


console.log(obj.d);

edited Jan 9, 2017 at 16:45

answered Jan 9, 2017 at 16:31

Ori Drori

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Comments

Nina Scholz · Accepted Answer · 2017-01-09 16:48:37Z

1

Beside getters, you could use an ES6 feature, Proxy

The Proxy object is used to define custom behavior for fundamental operations (e.g. property lookup, assignment, enumeration, function invocation, etc).

var obj = {
        a: 0,
        b: 5,
    },
    p = new Proxy(obj, {
        get: function(target, prop) {
            return prop === 'c' ? target.a + target.b : target[prop];
        }
    });
   
console.log(p.c); // 5

edited Jan 9, 2017 at 16:48

answered Jan 9, 2017 at 16:38

Nina Scholz

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Comments

kemiller2002 · Accepted Answer · 2017-01-09 16:31:41Z

0

You can use a getter:

var o = {
   a : 0; 
   b : 1;
   get c () {return this.a + this.b;}

}

answered Jan 9, 2017 at 16:31

kemiller2002

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