Khan Academy Computer Science

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A talk on Khan Academy's Computer Science curriculum.

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Khan Academy Computer Science

  1. Khan AcademyComputer Science John Resig (ejohn.org) http://khanacademy.org/cs
  2. Inspiration• Bret Victor • http://vimeo.com/36579366
  3. Learning to Program• Learn through hands-on experimentation• Learn by looking at other people’s code• Learn by building on other people’s code
  4. CS Projects “Forked” New• “Forked”: 62930• New: 22225 26%• 1.86m Views 74%
  5. Github in Disguise• Forking (Hitting “Save as...”)• Versioning (Not yet user exposed)• Runtime Versioning • All runtime changes are versioned to protect against API changes
  6. Implementation• Graphics: Processing.js• Real-time Execution • Dynamic Injection • Error handling
  7. Graphics• Use Processing.js’ API• Works in all browsers with <canvas>• Import it wholesale, ignore the “Processing language” bits
  8. Real-Time Execution• Code is constantly checked for changes• If a change is detected, code is re-evaluated• Code is not run using a simple “eval”, code is dynamically injected into the current runtime.
  9. How Code is Injected• JSHint (run in worker thread) • Optionally run BabyHint • If Error, stop execution• Cache Images• Run Code in Worker Thread, Looking for long-running code • If error, stop execution• Execute Code • If first time running code, just eval • If later: • Eval code and extract current state • Inject extracted values into runtime • Maintain closures with functions!
  10. Error Messages• All code is run through JSHint• And through an extra layer of error handling (called “BabyHint”)
  11. JSHint• Validation!• Also: Gives us a list of all global variables used in the program• Compare against a master list of properties in Processing.js• Everything else is user-defined!
  12. BabyHint• Handles common mistakes: • Spelling and case: “strokeWeight” vs. “strokeweight” • Gives sane errors about missing semicolons • Provide hints about correct function arguments strokeWeight(); (gives an error asking for more args) • Check for function declaration mistakes and possible spacing mistakes (“vartest”)
  13. Worker Threads• Run JavaScript code asynchronously in the background of a page• Available in Chrome, Firefox, Safari, and IE 10 (Need to make sure it works in IE 9!)• Works by doing a string-only postMessage to the worker and waiting for a response
  14. With Statements• var obj = { name: “John”, city: “Boston” }; with (obj) { name += “ Resig”; city = “Brooklyn”; }• obj.name === “John Resig” obj.city === “Brooklyn”
  15. With Statements• var obj = { name: “John”, city: “Boston” }; with (obj) { var city = “Brooklyn”; var job = “Khan Academy”; }• obj.city === “Brooklyn” obj.job === undefined
  16. With Statements• var obj = { name: “John”, city: “Boston” }; obj.job = undefined; with (obj) { var city = “Brooklyn”; var job = “Khan Academy”; }• obj.city === “Brooklyn” obj.job === “Khan Academy”
  17. Example• var x = 5, y = 1; var draw = function() { x += y; };• JSHint: PASS• Long Run Detection: PASS• First Run: True • Code is evaluated • lastGrab = { x: “5”, y: “1”, draw: “function() { x += y; }” }
  18. Example (cont.)• var x = 50, y = 1; var draw = function() { x += y; };• JSHint: PASS• Long Run Detection: PASS• First Run: False • Code is evaluated • grabAll = { x: “50”, y: “1”, draw: “function() { x += y; }” } • Compare with lastGrab: grabAll.x !== lastGrab.x • Eval: “var x = 50;”
  19. Example (cont.)• var x = 50, y = 1; var draw = function() { x += y * 2; };• JSHint: PASS• Long Run Detection: PASS• First Run: False • Code is evaluated • grabAll = { x: “50”, y: “1”, draw: “function() { x += y * 2; }” } • Compare with lastGrab: grabAll.draw !== lastGrab.draw • Eval: “var draw = function() { x += y * 2; };”

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