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Google App Script Lesson for Students: Change Background based on Word Count.

Inspired by a request from +Mark Phillips.   Feel free to use this or a variation of this with your students.   Please do not republish a public version that includes my "Working Example" without my permission.


  • Have the class work together to create a Google Document App Script that will automatically change the background color of the document if it reaches a certain number of words. 

Working Example: Word Limit Notifier

  • If you would like to see the full code, then create a copy, and go to Tools => Script Editor.
  • There is a Group Tasks file that includes completed examples of each groups scripts. 
  • Learning how to code is best done by looking at other examples.  However there is always a thin line between looking at other's code to learn and taking it all-to-gather.   I suggest leaving it up on the teachers machine for students to reference.   Just extrapolating the information that is pertinent to them is an important skill in coding. 


  • As a Group create a function.
    • function getCharacterLimit(){
      var labelTxt = "The length: ";
      var ad = DocumentApp.getActiveDocument();
        var text = ad.getBody().getText();
        var lenghth = text.length;
  • Split the class into groups; each group is responsible for their section of the script.
  • Sections / Groups
    • User Interface: This group is responsible for creating the menu, dialogues and prompts.  The information they capture will need to be saved as a variable.
      • Prompt: "Set Count"  ~ log the result
      • Prompt: "Set bgColor" ~ log the result
      • Run: "Force Check"
      • Alert: "Current Count" ~ Multiline alert that will eventually include "Max Allowed Words:",  "Background Color:", "Word Count:".
      • Take it further: Use HtmlService instead of getUi to create the dialogue.
      • Documentation
    • Save hidden information: This group will be charged with learning how to save variable information using the PropertiesService so that it can be used the next time the document is opened.
      • Save the Set Count
      • Save the Set bgColor
      • Retrieve the Set Count
      • Retrieve the bgColor
      • Take it further: Learn to save information as an object using JSON.stringify() and JSON.parse().
      • Properties Service Documentation
    • Triggers:  Triggers tells the script to do something at a particular time or interval. In the scripts case it will be checking the word count and if it is above the set count then it will change the background.
      • Set a trigger to run a function that adds to the log every minute: (1 minute is the minimum amount of time a trigger can launch consecutively. 
      • Check to see if the trigger has already been installed.
      • Find out at what point the trigger should be installed in the completed script.
      • Take it further: Set date triggers, and learn about simple vs programmed triggers.
      • Trigger Documentation
    • Get the Word Count & Change the Background Color:  There is no program key that gives you the word count, so to complete this you will need to think of ways to accomplish getting at least a rough estimate of how many words.  Secondly you will want to be able to change the background color of the document.
      • Get the current word count and write it to the log
      • Take it further: Capture other information like character count or how many times the word "The" appears and see if you can log that information also.
      • Write a function that will change the background color.
      • Take it further: Find other attributes that you can change.
      • Documentation
  • Bring it together: 
    • In a class discussion format, or individually as homework have the students take all the different parts and put them together into a single script.
    • Take it further: Add in checks to prevent user error.
    • Take it further: Improve the user interface to make it simpler for the end user.

Glossary Of Terms:
  • Function: A section of code that can be run by itself.  
  • Variable: A word or letter that information gets stored to so that it can be accessed within a function.
  • Trigger: An event that fires at a particular time or interval.
  • String: Information stored as plain text. (var aString = "";)
  • Object: Used for storing information in a complex format. (var someObject = {};)
  • Array: This is a group of information that can be accessed one after another. (var anArray = [];)
  • API: The different commands that can be run to build your script.
  • Comment: A section of the code that does not run.
  • Log: A place to store information while you are developing. 

Import things to know:
  • Each link of code ends with a semi-colon.  A line of code may take up severs lines on the document, but when it is read by the application, it looks for the semi-colon to know when it actually ends.
  • Functions have a format of:   function functionName(){  code goes here }
    • Variables can be passed between functions, by placing them inside the ( )'s.
  • Periods are used to add a sub-API event to the root API.  DocumentApp.getUi();
  • Save and test often.
  • Most of App Scripting is based off of Javascript.

Important Links:
  • API reference ~ Primary documentation that lists of all the commands that can be run (Also found in the Help menu of the script)
  • StackOverFlow.com ~ Scripting forum used to get or provide help with scripts. 
  • Issue Tracker ~ Used to list any bugs found in the API's.

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