Javascript Object watch

Question

I use the next code for watch changes in javascript object: https://gist.github.com/eligrey/384583

I have a next code for check updates:

var Obj = {
    prop: 0,
    inc: function() {
      this.prop ++;
    }
};

Obj.watch('prop', function(prop, oldv, newv) {
    console.log(prop);
    console.log(oldv);
    console.log(newv);
  });

// on button click i increment prop:

$("#btn").on('click', function() {
    Obj.inc();
});

When i run this code, and click button i get in console:

prop
0 
1

It's correctly.

But, when i click the next time, i get:

z
undefined
NaN

Online demo: http://jsbin.com/jenarapufo/2/edit?html,js,console,output

Why?

because u forgot to return new value from watch callback add return newv; — Evgeniy
– Evgeniy, Commented Dec 12, 2014 at 12:02

Evgeniy · Accepted Answer · 2014-12-12 12:12:37Z

2

You need to return new value from watch callback. Update Your handler:

Obj.watch('prop', function(prop, oldv, newv) {
    ...
    return newv;
})

answered Dec 12, 2014 at 12:12

Evgeniy

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Comments

James Hibbard · Accepted Answer · 2014-12-12 12:17:14Z

1

You'll need to return the new value from the callback:

Obj.watch('prop', function(prop, oldv, newv) {
  console.log(prop);
  console.log(oldv);
  console.log(newv);
  return newv;
});

edited Dec 12, 2014 at 12:17

answered Dec 12, 2014 at 12:06

James Hibbard

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