Using JavaScript to access a JSON array from PHP

Question

I'm trying to understand how my PHP script can pass an array to my JavaScript code. Using the following PHP, I pass an array:

$c = array(3,2,7);

echo json_encode($c);

My JavaScript is as follows:

    $.post("getLatLong.php", { latitude: 500000},
        function(data) {
            arrayData = data
            document.write(arrayData)
            document.write(arrayData[0]);
            document.write(arrayData[0]);
            document.write(arrayData[0]);
        });
</script>

What is printed out on the screen is

[3,2,7][3,

I'm trying to understand how json_encode works - I thought I would be able to pass the array to a variable, and then access it like a normal JavaScript array, but it views my array as one large text string. How do ensure that it reads it like an array?

Your example is a 2-dimensional array. Try using data[0][0], [0][1],[0][2], etc. to access 3,2,7 respectively. — Brad Christie
– Brad Christie, Commented Jan 15, 2011 at 20:27
hmm, I understand now that it is a 2D array. but even if I change it, it still reads it as a string. — djq
– djq, Commented Jan 15, 2011 at 20:29
@Brad Christie: It is not at all a 2-dimensional array. What makes you think that? — Felix Kling
– Felix Kling, Commented Jan 15, 2011 at 20:41
@Felix: Look a their revision history. Their first code implemented (what's being referred to as $c) as array(array(3,2,7)). But, I take full accountability for missing that they didn't specify they wanted JSON parsed (though I can't confirm if they included a header() in the AJAX page specifying JSON that .post wouldn't take care of it automatically) — Brad Christie
– Brad Christie, Commented Jan 15, 2011 at 20:44

Felix Kling · Accepted Answer · 2011-01-15 20:46:13Z

4

Pass the dataType argument to $.post:

$.post("getLatLong.php", { latitude: 500000},
   function(data){         
   // ...
}, 'json');

Then data will be properly decoded.

Alternatively you can also use $.parseJSON in the callback.

Explanation of JSON:

JSON is a data exchange format. It specifies the structure of the data string that is transmitted. json_encode takes a PHP data structure like an array or an object and transformes it to JSON, i.e. a string with a certain format.

json_encode($c) will give you the string "[3,2,7]".

On the client site, you receive that string, but you have to decode it in to proper JavaScript data structures. So $.parseJSON(data) will give you a JavaScript array with 3 elements (or if you pass 'json' as third parameter to .post(), jQuery is doing the decoding automatically for you).

edited Jan 15, 2011 at 20:46

answered Jan 15, 2011 at 20:34

Felix Kling

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4 Comments

Brad Christie Over a year ago

As it turns out, if they placed header('Content-Type: application/json') it appears .post would have worked correctly (or at least not needed to be explicitly noted it's a JSON reply [which makes sense])

djq Over a year ago

@Brad where would I place the header? Just trying to understand this overall process... as the code is now working.

TH1981 Over a year ago

place the header in getLatLong.php

Felix Kling Over a year ago

@celenius: ... before any output.

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