Having problems with d3.json, doesn't work

Question

I'm fairly new to both Javascript and D3, and I've been trying to try the examples from the website myself.

I used the following for the JS/HTML code:

<!DOCTYPE html>
<html>
    <head>

        <script src="http://d3js.org/d3.v3.min.js"></script>

        <style type="text/css">

        </style>

    <head>
    <body>
        <script>
        d3.json("mydata.json", function (data) { 
            var canvas = d3.select("body").append("svg")
            .attr("width", 500)
            .attr("height", 500)

            canvas.selectAll("rect")
                .data(data)
                .enter()
                .append("rect")
                .attr("width", function (d) { return d.age * 10; })
                .attr("y", function (d, i) { return i * 50; })
                .attr("fill", "blue")
        })
           </script>
    </body>
</html>

and for my "mydata.json" I am using:

[
    {"name": "Maria",   "age": 30},
    {"name": "Fred",    "age": 50},
    {"name": "Jason",   "age": 12}  
]

Every-time I try and run it, it doesn't have the same result as the examples on the d3 examples page. Please help, I am try to figure out how d3 works, and I am quite a novice programmer.

What do you mean when you say that it doesn't have the same result? Note that you cannot load local files using d3.json, see e.g. this question. — Lars Kotthoff
– Lars Kotthoff, Commented Sep 21, 2013 at 16:09
Not sure what you're expecting, but your code sets the width attribute of the <rect> elements but not the height; similarly, it sets the y position but not the x position. Without those properties, the results won't be very interesting. — Stephen Thomas
– Stephen Thomas, Commented Sep 21, 2013 at 18:41

Christopher Chiche · Accepted Answer · 2013-09-21 23:43:34Z

It seems like you have to set the height attribute to display something.

A good way of debugging this kind of problem is to first make everything work without the json call. To do so I would recommend you to use the chrome javascript console to display errors and the chrome inspector to see the html code.

data = [{
    "name": "Maria",
    "age": 30
}, {
    "name": "Fred",
    "age": 50
}, {
    "name": "Jason",
    "age": 12
}]
//d3.json("mydata.json", function (data) { 
var canvas = d3.select("body").append("svg")
    .attr("width", 500)
    .attr("height", 500)

canvas.selectAll("rect")
    .data(data)
    .enter()
    .append("rect")
    .attr("width", function (d) {
        return d.age * 10;
    })
    .attr("height", "20px")        
    .attr("y", function (d, i) {
        return i * 50;
    })
    .attr("fill", "blue")
//})

jsFiddle: http://jsfiddle.net/chrisJamesC/LX9BF/

Community · Accepted Answer · 2017-05-23 12:21:26Z

As stated here, the problem stems from a security restriction when using file:// style URLs.

One solution, other than embedding the JSON object in your .js file, is to load the files via http. This is easily achieved using nodejs!

Follow these steps:

Install nodejs.
Open a command prompt, then cd to your demo folder.
Type the command: npm install Express. This will install the Express web-server components into the .\node_modules folder.
Create a new web-server.js file with the contents shown below.
Invoke the server by typing node .\web-server.js on the command-line.
Create a sub-folder called htdocs then move your .html, .js, and .json files, and any other web assets into it. This will be the root folder of your web site.
Open a browser and navigate to http://localhost:9999/[your-html-file].html, where [your-html-file] is replaced with the actual filename.

web-server.js

var server_port = 9999;
var application_root = __dirname + "\\htdocs",
    express = require("express"),
    path = require("path");
var app = express();
app.use(express.static(application_root));
app.use(app.router);
app.listen(server_port);
console.log("Web server listening on port " + server_port + ".");

This should get you past the issue of loading the .json file. In fact, it's useful whenever you need a light-weight web server to test code with.

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