7

I have this interval, that does an ajax request, currently every 5 seconds. I am having an issue with the if statement. my code ALWAYS enters it, and the two json values are the exact same, why is it seeing them as different?

var newActivity = null, oldActivity = null;
setInterval(function(){
    $.ajax({
        type: "get",
        url: "/get/new_activity",
        dataType: "json",
        success: function(data){
            oldActivity = newActivity;
            newActivity = data;
            console.log(JSON.stringify(oldActivity));
            console.log(JSON.stringify(newActivity));
            if(JSON.stringify(oldActivity) != JSON.stringify(newActivity)){
                $("#new-activity").slideDown( "fast" );
            }
        }
    });
}, 5000);

Edit
Here is the console output (dashed line is to separate requests, it isn't in the actual output)

null
[{"title":"How many planets are in the solar system?","title_url":"How-many-planets-are-in-the-solar-system%3F","id":"2","answers":"1","asked":"2013-01-11 10:03:50","asked_pretty":"Today","activity":"2013-01-11 12:33:53","activity_pretty":"Today"},{"title":"Why is the sky blue?","title_url":"Why-is-the-sky-blue%3F","id":"1","answers":"1","asked":"2013-01-11 09:55:13","asked_pretty":"Today","activity":"2013-01-11 12:03:45","activity_pretty":"Today"}]

---------------------------------------------------

[{"title":"How many planets are in the solar system?","title_url":"How-many-planets-are-in-the-solar-system%3F","id":"2","answers":"1","asked":"2013-01-11 10:03:50","asked_pretty":"Today","activity":"2013-01-11 12:33:53","activity_pretty":"Today"},{"title":"Why is the sky blue?","title_url":"Why-is-the-sky-blue%3F","id":"1","answers":"1","asked":"2013-01-11 09:55:13","asked_pretty":"Today","activity":"2013-01-11 12:03:45","activity_pretty":"Today"}]
[{"title":"How many planets are in the solar system?","title_url":"How-many-planets-are-in-the-solar-system%3F","id":"2","answers":"1","asked":"2013-01-11 10:03:50","asked_pretty":"Today","activity":"2013-01-11 12:33:53","activity_pretty":"Today"},{"title":"Why is the sky blue?","title_url":"Why-is-the-sky-blue%3F","id":"1","answers":"1","asked":"2013-01-11 09:55:13","asked_pretty":"Today","activity":"2013-01-11 12:03:45","activity_pretty":"Today"}]

---------------------------------------------------

[{"title":"How many planets are in the solar system?","title_url":"How-many-planets-are-in-the-solar-system%3F","id":"2","answers":"1","asked":"2013-01-11 10:03:50","asked_pretty":"Today","activity":"2013-01-11 12:33:53","activity_pretty":"Today"},{"title":"Why is the sky blue?","title_url":"Why-is-the-sky-blue%3F","id":"1","answers":"1","asked":"2013-01-11 09:55:13","asked_pretty":"Today","activity":"2013-01-11 12:03:45","activity_pretty":"Today"}]
[{"title":"How many planets are in the solar system?","title_url":"How-many-planets-are-in-the-solar-system%3F","id":"2","answers":"1","asked":"2013-01-11 10:03:50","asked_pretty":"Today","activity":"2013-01-11 12:33:53","activity_pretty":"Today"},{"title":"Why is the sky blue?","title_url":"Why-is-the-sky-blue%3F","id":"1","answers":"1","asked":"2013-01-11 09:55:13","asked_pretty":"Today","activity":"2013-01-11 12:03:45","activity_pretty":"Today"}]
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18
  • 1
    Please post the actual values resulting from the JSON stringification of newActivity and oldActivity. If they contain objects then there is no guarantee of ordering or equality in JSON form. (The are other encodings that are equivalent but not identical; they are less common.) Commented Jan 11, 2013 at 21:21
  • 1
    @Christophe Except it was not correct at all, given with the first output listing. Commented Jan 11, 2013 at 21:27
  • 1
    @Christophe pst is correct, I deleted my answer because it was wrong. Commented Jan 11, 2013 at 21:29
  • 2
    @Christophe: The fact that his code says they are different. Commented Jan 11, 2013 at 21:35
  • 2
    I am an idiot. I reset newActivity to null in a different function. That is why they were coming back as not equal every time. Commented Jan 11, 2013 at 21:41

3 Answers 3

Reset to default
10

JSON objects are not guaranteed to be serialized the same way, properties are not guaranteed to be in the same order, using JSON.stringify is not a good way to test object equality.

A better example is a function like (found on the internet a while ago, wish I could give credit to the original author)

/**
 * Deep compare of two objects.
 *
 * Note that this does not detect cyclical objects as it should.
 * Need to implement that when this is used in a more general case. It's currently only used
 * in a place that guarantees no cyclical structures.
 *
 * @param {*} x
 * @param {*} y
 * @return {Boolean} Whether the two objects are equivalent, that is,
 *         every property in x is equal to every property in y recursively. Primitives
 *         must be strictly equal, that is "1" and 1, null an undefined and similar objects
 *         are considered different
 */
function equals ( x, y ) {
    // If both x and y are null or undefined and exactly the same
    if ( x === y ) {
        return true;
    }

    // If they are not strictly equal, they both need to be Objects
    if ( ! ( x instanceof Object ) || ! ( y instanceof Object ) ) {
        return false;
    }

    // They must have the exact same prototype chain, the closest we can do is
    // test the constructor.
    if ( x.constructor !== y.constructor ) {
        return false;
    }

    for ( var p in x ) {
        // Inherited properties were tested using x.constructor === y.constructor
        if ( x.hasOwnProperty( p ) ) {
            // Allows comparing x[ p ] and y[ p ] when set to undefined
            if ( ! y.hasOwnProperty( p ) ) {
                return false;
            }

            // If they have the same strict value or identity then they are equal
            if ( x[ p ] === y[ p ] ) {
                continue;
            }

            // Numbers, Strings, Functions, Booleans must be strictly equal
            if ( typeof( x[ p ] ) !== "object" ) {
                return false;
            }

            // Objects and Arrays must be tested recursively
            if ( !equals( x[ p ],  y[ p ] ) ) {
                return false;
            }
        }
    }

    for ( p in y ) {
        // allows x[ p ] to be set to undefined
        if ( y.hasOwnProperty( p ) && ! x.hasOwnProperty( p ) ) {
            return false;
        }
    }
    return true;
},
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1 Comment

I wrote a simpler solution but with the same idea in this answer: stackoverflow.com/a/33378418/3060087
0

One solution would be to read the json data as text instead of json, and on success you check if the response string from the server differs from the last response string. Then you can call JSON.parse on the string. But this makes the comparison much easier and should only need a slight modification of your code.

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1 Comment

But dataType is set to "json", which means data is an object in your case, which makes comparison difficult. My suggestion is to set dataType to "text", and then skip JSON.stringify when comparing.
-2

I think you should try comparing their properties. 


for(var property in JSON.stringify(oldActivity))
    {
      if(JSON.stringify(oldActivity)[property]!=JSON.stringify(newActivity)[property])
                   $("#new-activity").slideDown( "fast" );
    }

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2 Comments

If you're going to be comparing properties, lose the JSON.stringify.
You're only pushing the stringify problem down one level, a recursive function is required, or you'll have to implement custom testing for each level of every JSON structure you ever compare

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