TypeError: xxx is not a function – Causes and Fixes [GAS / JavaScript]

When writing JavaScript or Google Apps Script (GAS), you may encounter an error like this:

TypeError: xxx is not a function

This article explains why this error occurs and how to fix it, with practical patterns that developers commonly encounter. We’ll focus on the pain points where GAS and JavaScript beginners often stumble, with solutions to prevent recurrence.

What Does TypeError: xxx is not a function Mean?

In short, this error means:

“You tried to call something as a function, but it isn’t actually a function.”

In JavaScript, the following syntax calls a function:

xxx();

However, if xxx is not a function but rather some other value (a string, number, array, object, undefined, etc.), this error occurs.

Common Cause #1: Overwriting a Function with a Variable

This is one of the most frequent causes.

Example: Defining a variable with the same name as a function

function getData() {
  return "data";
}
getData = "hello";
getData();

In this case, getData has become a string partway through. When you execute getData(), you get:

TypeError: getData is not a function

How to Fix It

• Never use the same name for a function and a variable
• Use const / let to maintain scope awareness

const result = getData();

Common Cause #2: Mistaking an Object Property for a Function

Example: You thought it was a function, but it’s a value

const user = {
  name: "Taro",
  age: 20
};
user.name();

name is a string, so it cannot be called as a function.

How to Fix It

• Before adding (), ask yourself “is this really a function?”
• Check the type with logging

Logger.log(typeof user.name);

Common Cause #3: Calling Arrays or Objects as Functions

Example: Calling an array like a function

const list = [1, 2, 3];
list();

Example: Calling an object like a function

const data = {};
data();

Both are typical causes of xxx is not a function.

How to Fix It

• Don’t add () to arrays or objects
• If you want to call a method, use .methodName()

list.push(4);

Common Cause #4: GAS-Specific “Return Value Confusion”

In GAS, return values often aren’t functions.

Example: Calling the result of getValues() as a function

const values = sheet.getRange("A1:A5").getValues();
values();

The return value of getValues() is an array.

How to Fix It

• Check the return type in the GAS official reference
• Verify with typeof or Array.isArray()

Logger.log(Array.isArray(values));

Common Cause #5: Function is Undefined When Called

Example: An expected function doesn’t exist

doSomething();

In reality, doSomething is never defined anywhere, or it’s not assigned in a conditional branch.

How to Fix It

• Add logging before execution
• Verify the function is actually defined

Logger.log(typeof doSomething);

Basic Debugging Checklist

When you encounter TypeError: xxx is not a function, check these points in order:

1. Is xxx actually a function?
2. Have you overwritten it as a variable somewhere?
3. Are you trying to call an array, object, or string?
4. Did you misunderstand the return type?
5. Is it defined outside the current scope?

These five points alone solve most cases.

Best Practices to Prevent This Error

// Separate function names from variable names
const / let for scope awareness
// Be mindful of return types
const result = getData();
// Make type-checking a habit
Logger.log(typeof fn);
// Use guard clauses
if (typeof fn === "function") {
  fn();
}

Guard clauses like this are very effective.

Summary

TypeError: xxx is not a function occurs because of assuming something is a function when it isn’t.

In GAS especially, just being aware of:

• Return value types
• Object structure
• Variable overwriting

can drastically reduce how often this error occurs.

Don’t get confused by the error message itself. The key to solving it is calmly asking: “What is this xxx that I’m trying to call?”