Elvenware

JsonBasics

Welcomes to JsonBasics

Overview

JSON, like XML, is a text base sepecification for exchanging data. JSON is based on JavaScript. In fact, it is a subset of JavaScript. In particular, it allows you to declare simple JavaScript objects that consist of name value pairs:

{
    "firstName": "George",
    "lastName": "Washington"
}

You can also declare simple JavaScript arrays:

[a, b, c]

You can combine the two:

[
    {
        "firstName": "George",
        "lastName": "Washington"
    }, {
        "firstName": "John",
        "lastName": "Adams"
    }, {
        "firstName": "Thomas",      
        "lastName": "Jefferson"
    }
]

Reading JSON with Node

There is a library called fs (FileSystem) that is built into Node. You can load the library like this:

var fs = require('fs');

Then you can use the library to read in a file:

var json = fs.readFileSync('index.json');

The code above reads in a text file and puts the contents in the variable of type string called json.

Here is how to do the same thing asynchronously:

fs.readFile('index.json', function(err, result) {
    if (err) {
        throw err;
    }
    var json = result;
});

Finally, you can convert that JSON string into a real JavaScript object by writing this code:

var obj = json.parse(json);

So your code might look like this:

var fs = require('fs');

function DoSomethingWithTheJson(json) {
    console.log(JSON.stringify(json, null, 4));
}

fs.readFile('./index.json', function(err, result) {
    if (err) {
        throw err;
    }
    var json = JSON.parse(result);
    DoSomethingWithTheJson(json);
});

The JSON.stringify method is the opposite of JSON.parse: it converts a JavaScript object into a string. In this example, we ask that no special substitutions are made (null) and that the code be formated with 4 spaces in the indentations. It's a bit ironic that we convert the data from a string to JavaScript object and then back again. That step is not strictly necessary in this example, but it is a very common procedure in this type of coding, so I provide an example of how to do it. Of course, in a real program the actions you perform with the JSON you read from file is likely to be very different than this. I'm simply trying to provide a relatively easy to understand "getting started" example. More advanced examples are available elsewhere.

Working with JSON

This code can go in the head section of your HTML:

<script type="textx-handlebars-template" id="par01">
    <p>{{text}}</p>
</script>

Here is code for using the template:

    function addItem(text) {  
        'use strict';  

        var script = \$("#par01").html(),  
        template=Handlebars.compile(script);  

        var result = template({  
            text: text  
        });  

        $("#myDiv").append(result);  
    }

    $.getJSON("index.json", function(data) {  
        $.each(data, function(i, president) {  
            $('#data01').append("<p>" + president.firstName
                + ' ' + president.lastName + "</p>");  
        });  
    });

You can download jQuery from http://jquery.com