How to Use Google Apps Script to Build Custom Functions for Google Sheets

The Limitation of Built-in Functions

Google Sheets comes with hundreds of built-in functions (like =SUM() or =VLOOKUP()). But occasionally, you encounter a highly specific mathematical or text-processing problem that requires chaining together five different formulas into a massive, unreadable block of code.

For example, if you need to calculate the shipping cost based on a complex matrix of weight, distance, and a proprietary corporate discount tier, a standard IF statement will become unmanageable.

Using Google Apps Script, you can write your own custom logic in JavaScript, give it a name, and then use it directly inside the Google Sheets formula bar, exactly as you would use a native function.

Step 1: Write the JavaScript Logic

1. Open your Google Sheet.
2. Click on Extensions > Apps Script.
3. Delete any default code.

We are going to create a simple custom function called DOUBLE_DISCOUNT. It will take a price, double it, and then apply a 20% discount.

Paste this code into the editor:

/**
 * Doubles a number and applies a 20% discount.
 *
 * @param {number} input The original price.
 * @return The final discounted price.
 * @customfunction
 */
function DOUBLE_DISCOUNT(input) {
  // Validate that the input is actually a number
  if (typeof input != 'number') {
    return 'Error: Input must be a number';
  }
  
  var doubled = input * 2;
  var discounted = doubled * 0.80; // 20% off leaves 80%
  
  return discounted;
}

The Importance of JSDoc Comments

Notice the block of comments at the very top, marked with /**. This is called JSDoc formatting.

The tag @customfunction is critically important. It tells Google Sheets to explicitly index this JavaScript function and make it available to the user in the spreadsheet’s autocomplete menu. The @param and @return tags will actually display as helpful tooltip instructions when a user starts typing the formula, mimicking the professional feel of native Google functions.

Click the Save (floppy disk) icon.

Step 2: Use the Function in Google Sheets

1. Return to your Google Sheet.
2. Type a number (e.g., 100) in cell A1.
3. Click in cell B1 and start typing: =DOUBLE_DISCOUNT(A1).

As you type, you will see Google Sheets recognize your custom function and display your help text. Press Enter, and the cell will display 160 (100 doubled is 200, minus 20% is 160).

Advanced Scenario: Fetching External Data

Custom functions are not limited to basic math; they can reach out to the internet to fetch live data.

Let’s build a custom function that converts USD to EUR using a live exchange rate API.

/**
 * Converts USD to EUR using live exchange rates.
 *
 * @param {number} usdAmount The amount in US Dollars.
 * @return The amount in Euros.
 * @customfunction
 */
function CONVERT_TO_EUR(usdAmount) {
  // Fetch live exchange rates from a public API
  var response = UrlFetchApp.fetch("https://api.exchangerate.host/latest?base=USD");
  var json = JSON.parse(response.getContentText());
  
  var euroRate = json.rates.EUR;
  var finalAmount = usdAmount * euroRate;
  
  return finalAmount;
}

Now, if you type =CONVERT_TO_EUR(50) in your spreadsheet, Google will instantly ping the API, fetch the exact exchange rate for that millisecond, perform the math, and output the result.

Conclusion

By leveraging Apps Script to build custom functions, you can hide massive complexity behind simple, readable commands. Whether you are embedding proprietary business logic or connecting cells to live web APIs, custom functions elevate Google Sheets from a simple calculator to a bespoke software application.