Strange behaviour in JSON.stringify with replacer function

Question

Consider this:

var object = {date: new Date()};

JSON.stringify(object, function(key, value) {
    console.log('---');
    console.log('key:', key);
    console.log('value:', value);
    console.log('value instanceof Date:', value instanceof Date);

    return value;
});

As documentation says:

The replacer parameter can be either a function or an array. As a function, it takes two parameters, the key and the value being stringified. The object in which the key was found is provided as the replacer's this parameter. Initially it gets called with an empty key representing the object being stringified, and it then gets called for each property on the object or array being stringified.

...

If you return any other object, the object is recursively stringified into the JSON string, calling the replacer function on each property, unless the object is a function, in which case nothing is added to the JSON string.

But if you run the code, you will get this:

---
key: 
value: { date: Fri Jan 10 2014 02:25:00 GMT+0100 (CET) }
value instanceof Date: false
---
key: date
value: 2014-01-10T01:25:00.262Z
value instanceof Date: false

It means, that the date property was stringified before replacer function has been called. Is it a normal behaviour or am I missing something? How can I affect format of Date stringification without overriding its default toJSON method?

Thanks!

Edit

Based on responses and next research, documentation seems to be unclear at this point and toJSON is beeing called before the replacer function. Based on Pills's response, this snippet should do the job:

var object = {date: new Date };

JSON.stringify(object, function(key, value) {
    if (typeof(value) === 'object') {
        for (var k in value) {
            if (value[k] instanceof Date) {
                value[k] = value[k].getTime();
            }
        }
    }
    return value;
});

Edit #2

Xotic750's solution is much better than previous one.

The docs do seem a little unclear, but it seems like stringification is done before the callback is made for individual properties. You could try something like this: http://jsfiddle.net/95eeS/ — Explosion Pills, Jan 10 '14 at 02:21
Looking at the source of [JSON2](https://github.com/douglascrockford/JSON-js/blob/master/json2.js#L240), any `toJSON` method gets called before the replacer. — Xotic750, Jan 10 '14 at 02:31

score 10 · Answer 1 · edited Feb 02 '23 at 13:21

10

Its quite old but just to complete it.

According to this Q/A and the MDN JSON.stringify article, the replacer is called with instance of the object in which the key was found so no need to change prototype or do another tricks:

function log(what) {
   what = what || "";
   document.getElementById("out").innerText += what + "\n";
}

function replacer(key, value) {
   console.log(this);
   log("Key: '" + key + "' = '" + value + "'");
   log("this = " + this);
   log("this[key] = " + this[key]);
   log("this[key] instanceof Date = " + (this[key] instanceof Date));
   log("this instanceof Date = " + (this[key] instanceof Date));
   
   if (this[key] instanceof Date) {
      return "This was a date: " + this[key].getTime();
   }
   
   return value;
}

var obj = {
   d: new Date()
};

var result;
result = JSON.stringify(new Date(), replacer);
log();
log(result);
log();
result = JSON.stringify(obj, replacer);
log();
log(result);

<pre id="out"></pre>

edited Feb 02 '23 at 13:21

Highmastdon

6,960
7
40
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answered May 13 '17 at 00:22

Fis

787
2
10
24

FYI, you can just use `console.log` in Stack Snippets and it will show the result. It will even show the object rather than `[object Object]`. – Heretic Monkey May 13 '17 at 01:12
I worked in fiddle so then I just copy pasted. But thanks for info. – Fis May 13 '17 at 03:26
This does seem more performant than deleting the prototype, but what makes this method better than iterating the object yourself (**Edit** in the question)? – Mihail Malostanidis Aug 10 '18 at 10:40
This is 100% the correct answer. I'm surprised this isn't more well-documented. – Qix - MONICA WAS MISTREATED Jun 12 '21 at 14:14

score 4 · Accepted Answer · answered Jan 10 '14 at 02:49

Perhaps you could do your formatting before passing the object to JSON, but otherwise here is a possibility:

JSON uses an objects toJSON method before calling the replacer.

So, before calling stringify store the toJSON method and then restore it afterwards.

var object = {
        date: new Date()
    },
    storeToJSON = Date.prototype.toJSON;

delete Date.prototype.toJSON;

JSON.stringify(object, function (key, value) {
    console.log('---');
    console.log('key:', key);
    console.log('value:', value);
    console.log('value instanceof Date:', value instanceof Date);

    return value;
});

Date.prototype.toJSON = storeToJSON;

On jsFiddle

Strange behaviour in JSON.stringify with replacer function

Edit

Edit #2

2 Answers2