javascript - Issues with Date() when using JSON.stringify() and JSON.parse()

Question

I am trying to calculate the difference between two times using JavaScript. It's just basic math but I seem to have some issues with that while using JSON.stringify() and JSON.parse().

If you're wondering why am I applying the JSON.stringify() function to the date, it's because I using local storage to store some data on the client side and use it whenever the client lands on my website again ( it's faster that way rather than making more requests to the server ). That data usually updates once in a while ( I'm grabbing the data through API from another website ), so I set up a data_update variable and I'm storing it together with the other data.

That way I'm grabbing the stored data from the local storage and check if the difference between data_update ( which is a date / time ) and the time / date when the check it's made and see if it's greater than a week / day /etc .

So that is the reason why I'm using the JSON functions. My problem is that when I'm parsing the data from the local storage, the date seems to be different from a Date() object.

I'm trying to do the next operation per say :

var x = JSON.parse(JSON.stringify(new Date()));

var y = JSON.parse(this.get_local_storage_data(this.data_cache_key)); // the data object stored on local storage

var q = y.data_update; // this is the variable where the Date() was stored

console.log(Math.floor((x-q)/1000));

The above will return null. Also when I want to see the Math.floor(x) result, it returns null again.

So what can I do in this situation ? Is there a fix for this ?

Answer 1

If you look at the output of JSON.stringify for a Date, you'll see that:

JSON.stringify(new Date())

Results in a string. JSON does not have a primitive representation of Date objects that JSON.parse will turn back into a Date object automatically.

The Date object's constructor can take a date string, so you can turn those string values back into dates by doing:

var x = new Date(JSON.parse(JSON.stringify(new Date())));

Then the arithmetic will work.

x = new Date(JSON.parse(JSON.stringify(new Date())))
y = new Date(JSON.parse(JSON.stringify(new Date())))
y - x
=> 982