How can I lazy evaluate fields on a javascript object?

Question

I am wondering if it is possible to do something like the following:

var obj = {
   counter: (function(){
                if(!this.val){ this.val = 0; }
                this.val += 1;
                return this.val;
            })();
};

console.log(obj.counter); //should output 1
console.log(obj.counter); //should output 2
console.log(obj.counter); //should output 3
...

Is there a way to get a field from an object like this such that it re-evaluates a function each time it is accessed?

Counter is being defined as an IIFE, so your reference to `this` is lost to the IIFE itself. Thus every time you call `obj.counter`, `this.val` will be undefined, and thus set to 0, then 1. @ksven beat me to the answer code, so am posting as a comment. — Kodlee Yin, Jun 26 '14 at 15:47
alright, can I put the actual counting variable elsewhere and still get the auto incrementing? — RutledgePaulV, Jun 26 '14 at 15:48

score 8 · Accepted Answer · answered Jun 26 '14 at 15:47

8

You can use a getter:

var obj = {};
Object.defineProperty(obj,"counter",{
    get: function() {
        this.val = this.val || 0;
        this.val++;
        return this.val;
    }
});

console.log(obj.counter); // 1
console.log(obj.counter); // 2
console.log(obj.counter); // 3

answered Jun 26 '14 at 15:47

Niet the Dark Absol

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This is great, I didn't know it existed. Thanks! – RutledgePaulV Jun 26 '14 at 15:51

georg · Answer 2 · 2014-06-26T15:52:59.203

This is possible with proxies if your target platform supports them:

var obj = Proxy.create({
    get: function(target, value) {
        if(value == 'counter')
            return this.val = (this.val || 0) + 1;
    }
});

console.log(obj.counter); //should output 1
console.log(obj.counter); //should output 2
console.log(obj.counter); //should output 3

Another option would be a getter:

obj = Object.create({}, {
    counter: {
        get: function() {
            return this.val = (this.val || 0) + 1;
        }
    }
})

or a valueOf object (this doesn't work with console.log, but does with arithmetic):

var obj = {
    counter: {
        valueOf: function() {
            return this.val = (this.val || 0) + 1;
        }
    }
};

console.log(obj.counter+5); // 6
console.log(obj.counter+5); // 7
console.log(obj.counter+5); // 8

Alan Turing · Answer 3 · 2019-02-17T07:57:13.683

Using Proxy class as well as use case for evaluate requrements for property evaluation

/* https://github.com/hack2root/lazyeval */

let lazy = (eval) => ((data) => new Proxy(data, {
  set(obj, key, val) {
  obj[key] = val;
  eval(obj);
  }
}))({});

// 1. ARRANGE
let a = 1;
let b = 2;
let c;

// 2. ACT
let func = lazy((f) => {
  if (f.a && f.b) { 
    c = f.a + f.b 
  }
});

func.a = a;
func.b = b;

// 3. ASSERT
console.log("c is", c);

let lazy_require = (requre) => (eval) =>  ((data) => new Proxy(data, {
    set(obj, key, val) {
        obj[key] = val;
        if (requre(obj)) {
            eval(obj);
        }
    }
}))({});

// 1. ARRANGE
let a_f = 1;
let b_f = 2;
let c_f;

// 2. ACT
let func_reqire = lazy_require((f) => f.a && f.b);

let lazy_func = func_reqire((f) => {
  c_f = f.a + f.b
});

lazy_func.a = a_f;
lazy_func.b = b_f;

// 3. ASSERT
console.log('c_f is', c_f);

let lazy_require_data = (requre) => (eval) => (data) => new Proxy(data, {
  set(obj, key, val) {
    obj[key] = val;
    if (requre(obj)) {
      eval(obj);
    }
  }
});

// 1. ARRANGE
let a_data = 1;
let b_data = 2;
let c_data;

// 2. ACT
let func_require_data = lazy_require_data((f) => f.a && f.b);

let func_data = func_require_data((f) => {
    c_data = f.a + f.b
});

let func_json = func_data({
    a: a_data,
    b: b_data
});

func_json.a = a;
func_json.b = b;

// 3. ASSERT
console.log('c_data is', c_data);

How can I lazy evaluate fields on a javascript object?

3 Answers3