Online SaaS Peer Assessment / Grading / Polling

Have spent several hours looking for a web-based tool that allows students to vote for each others presentations; by the best free solution was

(p.s. last year spent half a day counting students votes; so decided to automate it this year).

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java.lang.NoSuchMethodError: com.amazonaws.transform.JsonErrorUnmarshaller: method (Ljava/lang/Class;)V not found


An exception or error caused a run to abort: com.amazonaws.transform.JsonErrorUnmarshaller

java.lang.NoSuchMethodError: com.amazonaws.transform.JsonErrorUnmarshaller: method <init>(Ljava/lang/Class;)V not found


AWS had some strangely maintained libraries for lambda

"com.amazonaws" % "aws-lambda-java-core" % AWSLambdaLibsVersion,
"com.amazonaws" % "aws-lambda-java-events" % AWSLambdaLibsVersion,
"1.1.0" //

The error was fixed by replacing above with:

"com.amazonaws" % "aws-java-sdk-lambda" % "1.11.109",


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No compatible version found: @types/react-dom@15.4.2


Was puzzled by the

No compatible version found: @types/react-dom@15.4.2

In the end chaning all of the react related vers to


did the trick

also had an error (2nd one bellow); but that was due to a server already running on the same port


npm install
▌ ╢░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░╟
▀ ╢░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░╟
npm ERR! Linux 4.4.0-59-generic
npm ERR! argv "/usr/bin/nodejs" "/usr/bin/npm" "install"
npm ERR! node v4.2.6
npm ERR! npm v3.5.2

npm ERR! No compatible version found: @types/react-dom@15.4.2
npm ERR! Valid install targets:
npm ERR! 0.14.21, 0.14.20, 0.14.19, 0.14.18, 0.14.17, 0.14.16, 0.14.15, 0.14.14, 0.14.13, 0.14.12-alpha, 0.14.11-alpha, 0.14.10-alpha, 0.14.9-alpha, 0.14.8-alpha, 0.14.7-alpha, 0.14.2-alpha, 0.14.1-alpha
npm ERR! 
npm ERR! 
npm ERR! If you need help, you may report this error at:
npm ERR! <>

npm ERR! Please include the following file with any support request:
npm ERR! /home/neil/MEGA/MEGAsync/docs/projects/patentBot/code/patentBotUI/npm-debug.log


npm start

> mobx-react-typescript-boilerplate@1.0.0 start /home/neil/MEGA/MEGAsync/docs/projects/patentBot/code/patentBotUI
> node server.js

 throw er; // Unhandled 'error' event

Error: listen EADDRINUSE
 at Object.exports._errnoException (util.js:870:11)
 at exports._exceptionWithHostPort (util.js:893:20)
 at Server._listen2 (net.js:1237:14)
 at listen (net.js:1273:10)
 at net.js:1382:9
 at GetAddrInfoReqWrap.asyncCallback [as callback] (dns.js:63:16)
 at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:82:10)

npm ERR! Linux 4.4.0-59-generic
npm ERR! argv "/usr/bin/nodejs" "/usr/bin/npm" "start"
npm ERR! node v4.2.6
npm ERR! npm v3.5.2
npm ERR! mobx-react-typescript-boilerplate@1.0.0 start: `node server.js`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the mobx-react-typescript-boilerplate@1.0.0 start script 'node server.js'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the mobx-react-typescript-boilerplate package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR! node server.js
npm ERR! You can get information on how to open an issue for this project with:
npm ERR! npm bugs mobx-react-typescript-boilerplate
npm ERR! Or if that isn't available, you can get their info via:
npm ERR! npm owner ls mobx-react-typescript-boilerplate
npm ERR! There is likely additional logging output above.

npm ERR! Please include the following file with any support request:
npm ERR! /home/neil/MEGA/MEGAsync/docs/projects/patentBot/code/patentBotUI/npm-debug.log





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React (TypeScript): infinite scroll

Recently I had to use an infinite scroll (lazy loading).  It took me a while to find a solution that worked well for my very simple case (lazy loading items in an infinite scroll fashion).  Hopefully this will save somebody time. ended up being by far the best solution that worked flawlessly  (and is extremely easy to use).  Just add it to the end of your scrollable list and lazy load once it becomes visible (p.s. you can move its position slightly upward to lazy load in a more eager manner):

    onChange={(isVisible) => isVisible ? this.loadMoreData() : console.log("debug: became invisible")}

p.s. make sure to use arrow functions otherwise you might have some problem with this binding etc.; more details:’this’-in-TypeScript#fixes

to avoid small amount of polling; you might also wan to set the `active` property to false; when your component is no longer visible.

Finally react-visibility-sensor could be used for other purposes other than infinite scroll – that requires to know whether your component is visible or not.


Other Libraries

Most of the problems with other libraries were due them estimating weather the component is visible or not based on the position of the scroll bar.  Some did by requiring you to specify the height of each of the component.  In part this was motivated by many of the infinite scroll packages aiming at the case were the number of items would be very large 10K+ (and would only keep a small portion of visible items; to improve memory usage).  In my case I had under 1,000 items (with images etc); so these types of solutions were not necessary.  None of the libraries bellow worked well for me (the firing events were not reliable).

Problem: didn’t update its position; so was stuck in the middle after more elements were added.


onEnter={() => console.log(“waypoint: on enter” )}
onLeave={() => console.log(“waypoint: on leave”)}
onPositionChange={() => console.log(“waypoint: on onPositionChange”)}

PROBLEM: `loadMore` often doesn’t fire 🙁

needed to `useWindow={false}` in order to get `loadMore` to fire


loadMore={() => console.log(“requested: loadMore ………………”)}
loader={<div className=”loader”>Loading …</div>}
(data) =>
<Item data={data} />

Other Notes

list for react:

Many of the infinite scroll libraries are aimed at efficiently rendering 1K+ results; that is not the case for me; since I mostly just need paging/on-demand fetching; so don’t need to add virualization managing (which complicates things).

a newer fork:
original (not updated for 3 years)

keywords: react simple non virtualized infinite scroll with page fetching loading lazy load



Other Notes

unrelated issue that I’ve experienced with is that having a property

height: window.innerHeight

would make the other component unresponsive/loose focus; removing that property fixed it.




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ReactJS: Uncaught TypeError: Cannot read property of undefined


You get: Uncaught TypeError: Cannot read property of undefined

This is due to react trying to render components that rely on variables that haven’t yet been set (quite a common occurence).


You can use conditional rendering shortcut to get rid of this annoyance; this is possible due to react treating undefined as false; so you can then add the following:

{this.props.MyComponentsVar && <MyComponent />}

p.s. this will only be rendered if this.props.MyComponentsVar is defined



keywords: react

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VM54310:600 Uncaught TypeError: _XXXXX2.default is not a constructor


VM54310:600 Uncaught TypeError: _XXXXX2.default is not a constructor


need to export default class e.g.:

export default class StateController { ...





keywords: typescript VM54310:600 Uncaught TypeError: _XXXXX2.default is not a constructor ts js webpack



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Gradual Migration from JavaScript to TypeScript


You have an existing project in which you’d like to use TypeScript.


Simply remove the outDir from tsconfig.json, e.g.:

  "compilerOptions": {
         "allowJs": false,
    "outDir": "./tsDist/",
    "module": "es6",
    "target": "es6",
    "experimentalDecorators": true,
    "moduleResolution": "node"
  "exclude": [

Everything should work now (since ts file will be compiled to a JS file at the same location; so all the imports etc will work just fine).

Sometimes things get out of sync between ts and js files (might have something to do with IntelliJ); in that case I just manually execute tsc command.

Alternative approach didn’t work well for me; e.g. due to the 100+ errors that would need to be resolved (when I simply renamed the js -> ts files).

keywords: mixing ts js typescript javascript in the same project incremental


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enabling decorators for mobx

mobx is really great for state management (I prefer it over redux)

it is even handier if you can use decorators; to do that its probably easier to use babel 6 (due to some of the issues described here).

using npm install –save you should end add the following packages:

 "babel": "^6.5.2",
 "babel-core": "^6.18.0",
 "babel-loader": "^6.2.7",
 "babel-plugin-transform-decorators-legacy": "^1.3.4",
 "babel-preset-es2015": "^6.18.0",
 "babel-preset-react": "^6.16.0",
 "babel-preset-stage-1": "^6.16.0",

and then just add the following to .babelrc

  "presets": [
  "plugins": ["transform-decorators-legacy"]







// —- ESNext syntax with decorators —-

ES6 decorators babel

Enabling decorators (optional) Decorators are currently a stage-2 ESNext feature. How to enable them

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webpack issues

may need to run ‘sudo npm install’ first

install missing dependencies with `sudo npm install <DEP>`
Module parse failed: /home/neil/Dropbox/w540-bak/bitbucket/sa-client-ui/src/app.js Unexpected token (199:40)
You may need an appropriate loader to handle this file type.
SyntaxError: Unexpected token (199:40)

This seems to fix above: `sudo npm install –save-dev babel-loader` as mentioned in

`Module not found: Error: Cannot resolve module ‘react’`

seems that packages have to be installed with `sudo nmp install …`

may need to run ‘sudo npm install’ first

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DynamoDB: An AttributeValue may not contain an empty string (Exception)


com.amazonaws.AmazonServiceException: One or more parameter values were invalid: An AttributeValue may not contain an empty string (Service: AmazonDynamoDBv2;


This cryptic message basically says that dynamodb is not able to save fields with empty strings (or null) (a rather inconvenient and artificial limitation); but doesn’t say which ones …

This post suggest a related solution that works quite nicely.

implicit class RichJsObject(original: JsObject) {
  def omitEmpty: JsObject = original.value.foldLeft(original) { 
    case (obj, (key, JsString(st))) if st.isEmpty => obj - key
    case (obj, (key, JsArray(arr))) if arr.isEmpty => obj - key
    case (obj, (_, _)) => obj

Then you can just apply it to your jsObj as: jsObj.omitEmpty

Often you may actually have a jsVallue; which could be converted to jsObj:[JsObject].omitEmpty.toString()


keywords: scala play json dynamodb empty null string insert

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