Intermediate Programming @Woodbury

Working with APIs


We know how to access, parse and modify JSON data. Now we’ll look at how we can use functions and API calls to access JSON data on the web.


  • used in asynchronous programming
  • when data is finally loaded, THEN call (run) another function


loadJSON("data.json", dataReceived);

function dataReceived(){
  //once data has been loaded, the callback code in here will run

Accessing JSON locally and via URLs

loadJSON("astros.json", dataReceived);


loadJSON("", dataReceived);

Your JSON file can be local or it can be on another server (at a different URL).

JSONP to solve cross-domain errors

Sometimes when working with JSON APIs you can get the following error

”’ XMLHttpRequest cannot load http://external-domain/service. No ‘Access-Control-Allow-Origin’ header is present on the requested resource. Origin ‘http://my-domain’ is therefore not allowed access. “’

  • a web browser may only allow scripts on page A to access data on page B if these two pages have the same origin
  • determined by url and port number
  • It prevents scripts from reading data from your domain and sending it to their servers
  • JSONP helps by pinging the server and returning JSON data wrapped in a function call with callback
  • much more detailed info can be found here
  • Info on using JSONP in P5JS can be found here


  • API = application programming interface
  • an interface for different pieces of software to communicate together over networks
  • created by orgs, programmers, companies to access their data
  • there are open APIs and those requiring authorization
  • start out by using APIs that have tutorials or sample code and consider trying out ones that don’t require authorization

Working with an API - the basics

An example - OpenWeatherMap

  • using the OpenWeatherMap API
  • Sign up and make a new account
  • Get your private API key


Do not post your API key online anywhere publicly! This includes on GitHub.

When working with APIs, they often have a specific way of constructing a URL to access specific data.


Multiple query values

These are name-value pairs.

How to access OpenWeatherMap

Construct the URL properly

  • Note: replace the APPID with your own ID

You can put this URL in your browser to see this data.

  • You may want to use the JSON Formatter Chrome extension to format this data and make it easier to read.
  • Or use Code Beautify JSON Viewer


  • Blog post with a few chosen easy-to-use open APIs
  • List of source data APIs that do not require authorization or credentials
  • Public APIs - a large list