Here is a list of projects that you can do to master Google App Script. If you don't know what Google App Script is, then in very simple terms, inside every Google drive files such as documents, spreadsheets, slides, forms, etc there is a place where you can write code and automate the tasks that you do frequently. Let's see some of the common such things and what Google Apps Script is offering to you.

1. Custom Functions.
When you write =VLOOKUP in a Google Spreadsheet and pass in some values to it, a function is getting executed and returning some values. The value that is being returned by the function is then shown as the current value of the cell where the function has been called. Similarly, we can write our own custom functions.
Suppose that your business is having a heavy calculation to compute the price of a product then you convert this logic to a function named 'PRICE' and then call it from anywhere in the sheet just like you call VLOOKUP.

For simplicity, let's take a simple example. Suppose that you have a report template. And the name of your report depends upon the name of the spreadsheet. If the spreadsheet name is 'July 2020' then the report name should be 'US SALES REPORT JULY 2020'. Every time you create a new report for the month then you need to change the report name at multiple places in the spreadsheet. Which can be tedious and is error-prone. The simplest way will be to write a custom function that can pull the spreadsheet name and update the report name everywhere.

  /* 
  * Populates the name of an active spreadsheet.
  *
  * @customfunction
  */ 
  function SPREADSHEETNAME(){
    try{
      return SpreadsheetApp.getActiveSpreadsheet().getName()
    }catch(err){
     return err.message
    }
  }

** Note: Functions can only change the value of the cell from where it has been called and it cannot change the value of a different cell **

2. Google Form notifications
You can get instant notification whenever there is a new response on Google Forms. There are already many add-ons available for it. But sometimes there might be restrictions in adding such addon inside your company mail account. In such cases, a script for yourself is much helpful.

  function runOnFormSubmit(e){  
   // All logics go here
  }

To get started, write a function as shown below. This will be the which is going to be triggered when new form submission has occurred. To do that go to menu> edit> current project triggered (Or you can click the trigger icon which is below the menu). This will open a new window that lists all the triggers that are available in the project. If you have started with a fresh document then this will be empty. Click on the '+ Add Trigger' button in the bottom right corner. And select the function runOnFormSubmit from the list, Under 'Select event source` choose `From form` and under `Select event type` choose `On form submit`.

Next, let's write some code.

 function runOnFormSubmit(e) { 
  try { 
   const template = "
         <div style="font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
           background-color: #E8EAF6;padding-bottom: .25rem;">
           <div style="max-width: 650px;box-sizing: border-box;border-width: 0;
             border-style: solid;margin:auto;">
             <div style="box-sizing: border-box;border-width: 0;border-style: solid;
              border-radius: .25rem;border-top-width: 8px;border-color: #9FA8DA;
              background-color: #3f51b5;font-weight: 200;font-size: 1.875rem;
              padding-top: 1.25rem;   padding-bottom: 1.25rem;color: #ffffff;
              text-align:center;">
              New Response
             </div>
            <div style="box-sizing: border-box;border-width: 0;border-style: solid;
             border-color: #e2e8f0;border-radius: .25rem;background-color: #ffffff;
             font-weight: 200;margin-top: .75rem;margin-bottom: .75rem;
             padding: 1.25rem;">
              <div style="box-sizing: border-box;border-width: 0;border-style: solid;
               border-color: #e2e8f0;font-weight: 500;font-size: 1rem">
               Hello, the form has received a new response
              </div>
            </div>
           </div>
         </div>"; 
   MailApp.sendEmail({ 
     to: 'to_1@mydomain.com, to_2@domain.com, to_3@domain.com', 
     subject: 'New Response', htmlBody: template 
   }) 
  } catch (err) { 
    //Do something with the error. 
  } 
}

So, what does the code do, It's first creating an HTML template that says "Hello, the form has received a new response" and using MailApp service, it sends emails to the list of people in the `to:`. You can change the list with one of yours.

3. Sheet watch
Sheet watch will be a function that runs every minute or every hour (Or the time interval that you specify). Sheet watch will look for a particular cell and if the value in that cell is changed to something that we are interested in, then the code will send an email to all the stakeholders. There is something similar to this in the Google spreadsheet called 'Notification rules' But this will be much more advanced.

function sheetWatch{
 try{
  const v = SpreadsheetApp
             .getActiveSpreadsheet()
             .getSheetByName('My Sheet')
             .getRange('A10')
             .getValue()
  if(v > 100){
    const template = "
     <div style="font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
      background-color: #E8EAF6;padding-bottom: .25rem;">
      <div style="max-width: 650px;box-sizing: border-box;border-width: 0;
        border-style:solid;margin:auto;">
        <div style="box-sizing: border-box;border-width: 0;
          border-style: solid;border-radius: .25rem;border-top-width: 8px;
          border-color: #9FA8DA;background-color: #3f51b5;font-weight: 200;
          font-size: 1.875rem;padding-top: 1.25rem;padding-bottom: 1.25rem;
          color: #ffffff;text-align:center;">
          Watch
        </div>
        <div style="box-sizing: border-box;border-width: 0;border-style: solid;
          border-color: #e2e8f0;border-radius: .25rem;background-color: #ffffff;
          font-weight: 200;margin-top: .75rem;margin-bottom: .75rem;
          padding: 1.25rem;">
          <div style="box-sizing: border-box;border-width: 0;border-style: solid;
            border-color: #e2e8f0;font-weight: 500;font-size: 1rem">
            Value at sheet My Sheet A10 has been changed.
          </div>
        </div>
      </div>"
   MailApp.sendEmail({
    to: 'to_1@mydomain.com, to_2@domain.com, to_3@domain.com',
    subject: 'Watch',
    htmlBody: template
   });
  }
 }catch(err){
  console.log(err.message)
 }
}

The above function will send a notification whenever the value in the cell is greater than 100.

Now, Here is a task for you. Think about how you can extend this function so that multiple cells and multiple values can be watched.