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18.12 JSON JSONP

JSONP is a method for sending JSON data without worrying about cross-domain issues.
JSONP does not use the XMLHttpRequest object.
JSONP uses the <script> tag instead.

JSONP Intro

JSONP stands for JSON with Padding.
Requesting a file from another domain can cause problems, due to cross-domain policy.
Requesting an external script from another domain does not have this problem.
JSONP uses this advantage, and request files using the script tag instead of the XMLHttpRequest object

<script src="demo_jsonp.php">

The Server File

The file on the server wraps the result inside a function call:

Example

<?php
$myJSON = '{ "name":"John", "age":30, "city":"New York" }';

echo "myFunc(".$myJSON.");";
?>

Show file

The result returns a call to a function named "myFunc" with the JSON data as a parameter.
Make sure that the function exists on the client.

The JavaScript function

The function named "myFunc" is located on the client, and ready to handle JSON data

Example 1: JSON JSONP - Request JSON using the script tag

function myFunc(myObj) {
  document.getElementById("demo").innerHTML = myObj.name;
}

Creating a Dynamic Script Tag

The example above will execute the "myFunc" function when the page is loading, based on where you put the script tag, which is not very satisfying.
The script tag should only be created when needed:

Example 2: JSON JSONP - Dynamic Script Tag

Create and insert the <script> tag when a button is clicked:

function clickButton() {
  var s = document.createElement("script");
  s.src = "demo_jsonp.php";
  document.body.appendChild(s);
}

Dynamic JSONP Result

The examples above are still very static.
Make the example dynamic by sending JSON to the php file, and let the php file return a JSON object based on the information it gets.

PHP file

<?php
header("Content-Type: application/json; charset=UTF-8");
$obj = json_decode($_GET["x"], false);

$conn = new mysqli("myServer", "myUser", "myPassword", "Northwind");
$result = $conn->query("SELECT name FROM ".$obj->$table." LIMIT ".$obj->$limit);
$outp = array();
$outp = $result->fetch_all(MYSQLI_ASSOC);

echo "myFunc(".json_encode($outp).")";
?>

PHP File explained:

Convert the request into an object, using the PHP function json_decode().
Access the database, and fill an array with the requested data.
Add the array to an object.
Convert the array into JSON using the json_encode() function.
Wrap "myFunc()" around the return object.

JavaScript Example

The "myFunc" function will be called from the php file:

function clickButton() {
  var obj, s
  obj = { table: "products", limit: 10 };
  s = document.createElement("script");
  s.src = "jsonp_demo_db.php?x=" + JSON.stringify(obj);
  document.body.appendChild(s);
}
function myFunc(myObj) {
  var x, txt = "";
  for (x in myObj) {
    txt += myObj[x].name + "
";
  }
  document.getElementById("demo").innerHTML = txt;
}

Try it yourself

Callback Function

When you have no control over the server file, how do you get the server file to call the correct function?
Sometimes the server file offers a callback function as a parameter:

Example 3: JSON JSONP - Callback Function

The php file will call the function you pass as a callback parameter:

function clickButton() {
  var s = document.createElement("script");
  s.src = "jsonp_demo_db.php?callback=myDisplayFunction";
  document.body.appendChild(s);
}

Try it yourself

JavaScript Fundamentals course home
Module 18. JS JSON

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Example files created in this module:
JSON Syntax - Access a JavaScript object part 1 JSON Syntax - Access a JavaScript object part 2 JSON Syntax - Modify a JavaScript Object part 1 JSON Syntax - Modify a JavaScript Object part 2 JSON Parse - Parsing JSON JSON Parse - JSON From the Server JSON Parse - Array as JSON JSON Parse - Parsing Dates part 1 JSON Parse - Parsing Dates part 2 JSON Parse - Parsing Dates part 3 JSON Stringify - Stringify a JavaScript Object part 1 JSON Stringify - Stringify a JavaScript Object part 2 JSON Stringify - Storing data in local storage JSON Stringify - Stringify Dates JSON Stringify - Stringify Functions part 1 JSON Stringify - Stringify Functions part 2 JSON Objects - Creating an Object from a JSON Literal part 1 JSON Objects - Creating an Object from a JSON Literal part 2 JSON Objects - Access a JavaScript Object part 1 JSON Objects - Access a JavaScript Object part 2 JSON Objects - Looping an Object part 1 JSON Objects - Looping an Object part 2 JSON Arrays - Create JavaScript Arrays JSON Arrays - Create JavaScript Arrays part 2 JSON Arrays - Accessing Array Values JSON Arrays - Accessing Array Values in Objects JSON Arrays - Looping Through an Array JSON Server - Sending Data to a Server JSON Server - Receiving Data from a Server JSON Server - JSON From a Server JSON Server - JSON returned from a server as an array: JSON Server - Get JSON Data from a PHP Server part 1 JSON Server - Get JSON Data from a PHP Server part 2 JSON Server - Get JSON Data from a PHP Server part 3 JSON Server - Get JSON Data from a PHP Server part 4 JSON Server - Get JSON Data from a PHP Server part 5 JSON HTML - Make a table based on JSON data JSON JSONP - Request JSON using the script tag JSON JSONP - Dynamic Script Tag JSON JSONP - Callback Function